Optical information recording medium and reproducing apparatus with layer identification

ABSTRACT

In an optical information recording medium having at least two information layer, guide grooves for tracking or sample pits or information pits corresponding to information signals are formed on a surface of a first substrate. A first information layer formed by a thin film for reflecting a portion of a light beam made incident on the first substrate and permitting penetration of a portion of the light beam is formed on a surface of the first substrate. Guide grooves for tracking or information pits corresponding to information signals are formed on a surface of a second substrate. A second information layer having a reflectance higher than that of the first information layer is formed on a surface of the second substrate. Between the first information layer and the second information layer, there is formed a transparent separation layer for positioning the first information layer and the second information layer to be spaced a predetermined distance apart from each other.

FIELD OF THE INVENTION

[0001] The present invention relates to an optical information recordingmedium capable of reproducing information signals or recording andreproducing information signals by using light beams, a manufacturingmethod therefor, a manufacturing apparatus therefor and an opticalinformation recording and reproducing apparatus making use thereof, andmore particularly to an optical information recording medium having amultilayer structure including a plurality of information layers, amanufacturing method therefor, a manufacturing apparatus therefor and anoptical information recording and reproducing apparatus making usethereof.

DESCRIPTION OF THE PRIOR ART

[0002] Hitherto, an optical information recording medium has been knownwhich is capable of optically recording information signals orreproducing recorded information signals and which is formed into anoptical disk, an optical card or the like. In general, the foregoingrecording medium uses a semiconductor laser unit as a light source. Byirradiating the recording medium with light beams finely converged by alens, information signals can be recorded on the recording medium in alarge quantity, and information signals recorded on the recording mediumcan be reproduced.

[0003] At present, investigations have been performed to further enlargethe recording capacity of the recording medium of the foregoing type. Toraise the recording density, it is effective to improve the reproducingresolution by finely limiting the light beam. In order to achieve this,investigations have been performed such that the wavelength of the lightbeam is shortened or the numerical aperture (NA) is enlarged. Moreover,reproducing methods have been investigated in which the focusing ortracking accuracy is improved and crosstalk between signals is preventedso as to allow the recording surface density to be raised.

[0004] Although employment of the foregoing methods enables therecording capacity per unit area to be somewhat enlarged, a structurehaving only a single information layer for recording informationencounters a limitation when the recording density is intended to beraised.

[0005] If a plurality of information layers for recording informationare provided, it can be expected that the recording capacity can bemultiplied. A method for manufacturing an optical disk having themultilayer structure has been disclosed in, for example, U.S. Pat. No.5,126,996.

[0006] A process for manufacturing the foregoing optical disk will nowbe described. As shown in FIG. 21(a), a first information layer 212 isformed on the surface of a substrate 211 manufactured by an injectionmolding method or the like and having information pits. Then, as shownin FIG. 21(b), a photosetting resin 214 is applied to the upper surfaceof a master 213 having information pits. Then, as shown in FIG. 21(c),the surface of the first information layer 212 on the substrate 211having the information pits and the surface of the master 213 having theinformation pits are caused to face each other. Then, in a state wherethe substrate 211 is pressed, the photosetting resin 214 is irradiatedwith light from a position on the outside of the master 213. As aresult, the photosetting resin 214 is set so that the photosetting resin214 is adhesive-bonded to the first information layer 212. Then, asshown in FIG. 21(d), the master 213 is removed from the photosettingresin 214. As a result, a resin layer having information pits on thesurface thereof and composed of the photosetting resin 214 can beformed. Then, as shown in FIG. 21(e), a second information layer 215 isformed on the resin layer (made of the photosetting resin 214). Finally,as shown in FIG. 21(f), a protective coating layer 206 is formed on thesecond information layer 215. As a result of the foregoing process, anoptical disk having a double-layer structure can be obtained.

[0007] However, when the master 213 is separated from the photosettingresin 214 (see FIG. 21(d)), the foregoing conventional manufacturingmethod can involves separation being easily taking place at theinterface between the first information layer 212 and the substrate 211or the resin layer (made of the photosetting resin 214). Thus, therearises a problem in that the manufacturing yield has beenunsatisfactory. The reason for this can be considered that theadhesivity between the master 213 and the resin layer (made of thephotosetting resin 214) is made to be greater than that between thefirst information layer 212 and the substrate 211 or that between thefirst information layer 212 and the resin layer (made of thephotosetting resin 214).

[0008] In a case where the substrate 211 is made of a resin, change inthe environmental temperature or in the humidity sometimes raises aproblem in that the manufactured optical disk can be deformed or anerror occurs in reproducing the signals.

[0009] Moreover, an apparatus of a type for reproducing informationsignals from a plurality of information layers has a problem in that theservo operation becomes instable due to an influence of light reflectedby an information layer other than the subject information layer.

SUMMARY OF THE INVENTION

[0010] The present invention was found to overcome the foregoingproblems experienced with the conventional structure, and, therefore, anobject of the present invention is to provide an optical informationrecording medium having a multilayer structure which cannot easily bedeformed due to change in the environment, a manufacturing methodtherefor, a manufacturing apparatus therefor and an apparatus foroptically recording and reproducing information.

[0011] To achieve the foregoing object, according to one aspect of thepresent invention, there is provided an optical information recordingmedium having two different information layers and capable ofreproducing information signals from the information layers, recordinginformation signals on the information layers that can reproduce therecorded information signals when irradiated with a light beam, theoptical information recording medium comprising: a first substratehaving, on one side thereof, one or more selected from the groupconsisting of information pits corresponding to information signals,guide grooves for tracking and sample pits; a first information layerformed on either surface of the first substrate and having apredetermined transmissivity and a predetermined reflectance withrespect to the light beam; a second substrate having, on one sidethereof, one or more selected from a group consisting of informationpits corresponding to information signals, guide grooves for trackingand sample pits; a second information layer formed on either surface ofthe second substrate and having a predetermined reflectance; and aseparation layer formed between the first information layer and thesecond information layer that is transparent with respect to the lightbeam. The structure of the optical information recording medium providesa double-layer recording medium that is capable of reproducinginformation signals from the first and second information layers,recording information signals on the first and second information layersand reproducing recorded information signals.

[0012] It is preferable that the optical information recording mediumaccording to the present invention has a structure such that thethickness of the first substrate is substantially the same as thethickness of the second substrate. As a result of the preferredstructure, a structure vertically symmetrical with respect to theseparation layer can be formed. Therefore, even if stress or the like isgenerated in the substrate due to change in the temperature or the likewhen the optical information recording medium is manufactured,distortion can be compensated. Even if unusual deformation factors acton the two substrates due to change in the temperature or the humidityof the environment, deformation and warp can be inhibited. As a result,a structure durable against change in the environment can be obtained.Therefore, even if the first and second substrates are made of resin,deformation of the manufactured recording medium causing an error inreproducing a signal can be prevented.

[0013] It is preferable that in the optical information recording mediumaccording to the present invention, the below mentioned formula issatisfied

R1≈1−A1+(2·R2)⁻¹−{[1−A1+(2·R2)⁻¹]²−(1−A1)²}^(0.5)

[0014] wherein R1 is a reflectance of the first information layer.

[0015] A1 is an absorption ratio of the first information layer, and

[0016] R2 is a reflectance of the second information layer with respectto the wavelength of the light beam used to reproduce the information.

[0017] It is preferable that the optical information recording mediumhas a structure such that the reflectance of the first information layeris 25% to 40%. As a result of the foregoing preferred structure, arecording medium having the first and second information layers, whichare reproduction only information layers, can be obtained in which theamplitudes of the signals from the first information layer and thosefrom the second information layer are similar to each other and thereproducing amplitude is satisfactorily large.

[0018] It is preferable that the optical information recording mediumaccording to the present invention has a structure such that the secondinformation layer includes a reflecting layer, a firstdielectric-material layer, a recording layer and a seconddielectric-material layer, which are sequentially laminated on thesecond substrate. According to the foregoing preferred structure, asecond information layer, the information of which can be rewritten, canbe obtained.

[0019] It is preferable that the optical information recording mediumaccording to the present invention has a structure such that the firstinformation layer has a coefficient of absorption which is substantiallyzero with respect to the light beam. According to the foregoingpreferred structure, the quantity of the light that reaches the secondinformation layer can be enlarged.

[0020] It is preferable that the optical information recording mediumaccording to the present invention has a structure such that the firstinformation layer includes at least two layers of thin films. Accordingto the foregoing preferred structure, the reflectance of the firstinformation layer can be enlarged and the quantity of the light thatreaches the second information layer can be enlarged. In the foregoingcase, it is preferable that the first information layer includes a firstdielectric-material layer, a recording layer and a seconddielectric-material layer, which are sequentially laminated on the firstsubstrate. According to the foregoing preferred structure, a firstinformation layer can be obtained on which information can be recordedor information of which can be rewritten.

[0021] It is preferable that the optical information recording mediumaccording to the present invention has a structure such that both of thefirst and second substrates have information pits corresponding toinformation signals on either surface. According to the foregoingpreferred structure, a recording medium having a large capacity can beobtained.

[0022] It is preferable that the optical information recording mediumaccording to the present invention has a structure such that the firstinformation layer is a reproduction only information layer and thesecond information layer is a recording and reproducing informationlayer. According to the foregoing preferred structure, light absorptionof the reproduction only first information layer can be set to a lowlevel. Moreover, since the influence of diffraction occurring due toinformation pits can be inhibited, information can be recorded orreproduced with small power.

[0023] It is preferable that the optical information recording mediumaccording to the present invention has a structure such that thethickness of the separation layer is larger than the focal depth of anoptical system for converging the light beam and smaller than thetolerance of a base allowed by the optical system. According to theforegoing preferred structure, information satisfactorily free fromcrosstalk can be reproduced from another information layer as long asthe aberration of the light beam to be applied is small.

[0024] It is preferable that the optical information recording mediumaccording to the present invention has a structure such that informationpits or sample pits are formed on the first and second substrates andthe shapes of the pits formed on the first substrate are different fromthe shapes of the pits formed on the second substrate. In the foregoingcase, it is preferable that the width of the pit formed on the secondsubstrate is larger than the width of the pit formed on the firs tsubstrate. According to the foregoing preferred structure, diffractionof light reflected from the first information layer and that from thesecond information layer can be made to correspond. Therefore, signalscan be reproduced stably.

[0025] It is preferable that the optical information recording mediumaccording to the present invention has a structure such that guidegrooves are formed in the surfaces of the first and second substratesand the width of the guide groove formed in the second substrate islarger than the width of the guide groove formed in the first substrate.According to the foregoing preferred structure, the effect of the guidegrooves formed in the surface of the first substrate and the effect ofthe guide grooves formed in the surface of the second substrate can bemade to be the same.

[0026] It is preferable that the optical information recording mediumaccording to the present invention has a structure such that informationpits are formed on the surfaces of the first and second substrates andthe density of the information pits formed on the second substrate perunit area is lower than the density of the information pits formed onthe first substrate per unit area. According to the foregoing preferredstructure, information can be reproduced satisfactorily from aninformation layer out of the focal depth of the optical system forconverging the light beam.

[0027] It is preferable that the optical information recording mediumaccording to the present invention has a structure such that sample pitsor guide grooves are formed in the surfaces of the first and secondsubstrates and the pitch of the sample pits or the guide grooves formedin the second substrate is shorter than the pitch of the sample pits orthe guide grooves formed in the first substrate. According to theforegoing preferred structure, a recording medium can be obtained inwhich signals can be reproduced satisfactorily from an information layerout of the focal depth of the optical system for converging the lightbeam and signals can be recorded on the same.

[0028] It is preferable that the optical information recording mediumaccording to the present invention has a structure such that informationpits or sample pits are formed on the surfaces of the first and secondsubstrates and the directions of the information pits are opposite toeach other between the first substrate and the second substrate whenviewed from a position upon which the light beam is made incident.According to the foregoing preferred structure, the material and moldingprocess of the substrates can be made to be the same as well as themastering process. As a result, it is only necessary to prepare twotypes of manufacturing apparatuses having the same function or onemanufacturing apparatus can be used commonly when the substrates aremanufactured. Thus, the cost of the facilities for manufacturing thesubstrates can be reduced.

[0029] It is preferable that the optical information recording mediumaccording to the present invention has a structure such that theinformation pits, the guide grooves or the sample pits formed on thesurfaces of the first and second substrates are formed into a spiralshape relative to the central portions of the first and second substrateand the shape of the spiral on the first substrate and the shape of thespiral on the second substrate are the same when viewed from a positionupon which the light beam is made incident. According to the foregoingpreferred structure, the light beam is moved in one direction from theinner portion to the outer portion or one direction from the outerportion to the inner portion regardless of the information layer havingthe information pits to which tracking has been made. In the case wherethe structure is employed in which the light beam is moved from theinner portion to the outer portion, a reproduction method may beemployed in which administration information is detected in the innerportion of any of the information layers and an access is made to arequired information region including the portion between theinformation layers. Therefore, it can be said that the foregoingstructure is suitable for a recording medium capable of allowing a highspeed access to the information layers.

[0030] It is preferable that a pair of the optical information recordingmediums according to the present invention and an adhesive-bonding layerare provided and the second substrates of the pair of opticalinformation recording mediums are adhesive-bonded to each other throughthe adhesive-bonding layer. According to the foregoing preferredstructure, a recording medium having a four-layer structure can beobtained in which information can be recorded and reproduced from eachof the information layers by irradiating the two sides with the lightbeams. In the foregoing case, it is preferable that the thicknesses ofthe first substrates of the pair of optical information recordingmediums are substantially the same and the thicknesses of the secondsubstrates of the pair of optical information recording mediums aresubstantially the same.

[0031] According to another aspect of the present invention, there isprovided a method of manufacturing an optical information recordingmedium having two different information layers and capable ofreproducing information signals from the information layers, recordinginformation signals on the information layers and reproducing therecorded information signals when irradiated with a light beam, Themethod of manufacturing an optical information recording mediumcomprises: a first film-forming step of forming a first informationlayer having a predetermined transmissivity and a predeterminedreflectance on a substrate having, on one side thereof, one or moreselected from the group consisting of information pits corresponding toinformation signals, guide grooves for tracking and sample pits; asecond film-forming step of forming a second information layer having apredetermined reflectance on a second substrate having, on one sidethereof, one or more selected from a group consisting of informationpits corresponding to information signals, guide grooves for trackingand sample pits; an applying step of applying a resin layer, which istransparent with respect to the light beam, to the upper surface of thefirst information layer or the second information layer; anadhesive-bonding step of causing the first information layer and thesecond information layer to face each other and adhesive-bonding thefirst and second information layers to each other through the resinlayer. According to the structure of the method of manufacturing theoptical information recording medium, the step of removing the master isnot required. By simply adhesive-bonding to each other the substrateshaving the information pits formed previously, a recording medium havinga double-layer structure can be obtained. As a result, the manufacturingyield can be improved.

[0032] It is preferable that the method of manufacturing an opticalinformation recording medium according to the present invention has astructure such that the thickness of the first substrate issubstantially the same as the thickness of the second substrate.

[0033] It is preferable that the method of manufacturing an opticalinformation recording medium according to the present invention furthercomprises a pressing step of pressing the first and second substratesfrom the outsides of the first and second substrates. According to theforegoing, a resin layer satisfactorily free from irregularity inthickness can be formed.

[0034] It is preferable that in the method of manufacturing an opticalinformation recording medium according to the present invention aphotosetting resin is used to form the resin layer and the resin layeris irradiated with light from a position on the outside of the firstsubstrate so as to set the resin layer while applying pressure to theoutsides of the first and second substrates. According to the foregoingpreferred structure, a resin layer satisfactorily free from irregularityin thickness can be formed in a short time.

[0035] It is preferable that the method of manufacturing an opticalinformation recording medium according to the present invention furthercomprises an applying step of applying an adhesive-bonding layer, whichis transparent with respect to the light beam, to the upper surface ofeither of the second substrates of a pair of optical informationrecording mediums obtained by the method of manufacturing an opticalinformation recording medium according to the present invention; and anadhesive-bonding step of causing the second substrates of the pair ofoptical information recording mediums to face each other andadhesive-bonding the second substrates to each other through the resinlayer. According to the foregoing, a recording medium having afour-layer structure can be obtained by repeating, three times, theadhesive-bonding step for obtaining the recording medium having thedouble-layer structure. That is, the recording medium having thefour-layer structure can be manufactured by a method similar to that formanufacturing the recording medium having the double-layer structure bybasically preparing the same manufacturing apparatuses for the threesteps. In the foregoing case, it is preferable that the thicknesses ofthe first substrates of the pair of optical information recordingmediums are substantially the same and the thicknesses of the secondsubstrates of the pair of optical information recording mediums aresubstantially the same.

[0036] According to another aspect of the present invention, there isprovided an apparatus for manufacturing an optical information recordingmedium comprising: a first substrate support section for supporting afirst substrate; a second substrate support section disposed to face thefirst substrate support section so as to support a second substrate; anapplying section for applying a resin layer to the upper surface of thefirst substrate or the second substrate; a spacer disposed on a plane onwhich the first substrate support section and the second substratesupport section face each other, the spacer being disposed outward fromthe first and second substrates; and a pressing section for elevatingthe first substrate support section or the second substrate supportsection and pressing the first and second substrates. The apparatus formanufacturing an optical information recording medium allows, arecording medium having a multilayer structure to be producedefficiently, and it can be expected that the recording capacity can beincreased.

[0037] It is preferable that the apparatus for manufacturing an opticalinformation recording medium according to the present invention furthercomprises a light source disposed opposite to a surface of the firstsubstrate support section that is in contact with the first substrate,wherein the first substrate support section is made of a material thatpermits penetration of a portion of light emitted from the light source.According to the foregoing preferred structure, the photosetting resinlayer is used so that the first substrate and the second substrate areadhesive-bonded to each other in a short time.

[0038] It is preferable that the apparatus for manufacturing an opticalinformation recording medium according to the present invention has astructure such that each of the first and second substrates has acentral opening in the central portion thereof and concentric or spiralprojection and pit columns or guide grooves on either surface thereof,and a center position correction section for making the central axes ofthe information pit columns or guide grooves of the first substrate andthe second substrate to coincide with each other is further provided forat least one of the first substrate support section and the secondsubstrate support section. According to the foregoing preferredstructure, a recording medium can be obtained in which the deviation ofthe circular arc of the information pit columns or the guide groovesformed on the surfaces of the two information layers can be inhibited.

[0039] It is preferable that the apparatus for manufacturing an opticalinformation recording medium according to the present invention furthercomprising first and second shaft sections respectively disposed on thecentral axes of the first and second substrate support sections; andfirst and second inner-portion guide sections each having a taperedportion having an end larger than the central opening of the substrateand another end smaller than the central opening of the substrate, thefirst and second inner-portion guide sections being capable of movingalong the first and second shaft sections. According to the foregoingpreferred structure, the first and second substrates can be secured tothe surfaces of the first and second substrate support sections in thestate where the tapered portions of the first and second inner-portionguide sections respectively are in contact with the central holes of thefirst and second substrates. Therefore, the central axis of the firstinformation layer and that of the first substrate support section can bemade coincide with each other with a value near the limit of mechanicalaccuracy. Moreover, the central axis of the second information layer andthat of the second substrate support section can be made coincide witheach other with a degree near the limit of mechanical accuracy. In theforegoing case, it is preferable that a projecting tapered section isformed on the leading end of one of the first shaft section or thesecond shaft section, and a corresponding recessed tapered portion isformed in the residual leading end of the other. According to theforegoing preferred structure, by moving the second substrate supportsection downwards, the projecting tapered portion formed on the leadingend of either of the first shaft section or the second shaft section andthe recessed tapered portion formed in the leading end of the other areengaged to each other. Thus, the central axis of the first informationlayer and that of the second information layer can be made to coincidewith each other.

[0040] According to another aspect of the present invention, there isprovided an apparatus for optically recording and reproducinginformation which irradiates, with light, an optical informationrecording medium having two different information layers provided withone or more selected from the group consisting of information pitscorresponding to information signals, guide grooves for tracking andsample pits so as to be capable of reproducing information signals fromthe information layers, recording information signals on the informationlayers and reproducing the recorded information signals. The apparatusfor optically recording and reproducing information comprises: opticalmeans for causing an objective lens to converge a light beam emittedfrom a light source onto the recording medium; focus control means forperforming control to make the focal point of the light beam coincidewith either of the information layers; tracking control means forcontrolling the position of the light beam to enable the light beam tofollow the information pits, guide grooves or the sample pits; layeridentification means for demodulating a signal for identifying theinformation layers in accordance with light reflected by or transmittedthrough the information pits; layer selection means for selecting aninformation layer from which the information signal is reproduced or onwhich the same is recorded; and switch means for switching a method oftracking, which is performed by the tracking control means, tocorrespond to a result of selection performed by the layer selectionmeans. According to the foregoing preferred structure of the anapparatus for optically recording and reproducing information,information can be recorded on and reproduced from the recording mediumhaving a multilayer structure.

[0041] It is preferable that the apparatus for optically recording andreproducing information according to the present invention furthercomprises layer comparison means for subjecting, to a comparison aresult of selection performed by the layer selection means and a resultof identification performed by the layer identification means, and afocus jumping circuit for generating a pulse voltage to move the focalpoint of the light beam between the information layers to correspond toan output from the comparison means. According to the foregoingpreferred structure, if an information layer, which is not the subject,is focused, the focusing position can be moved to the subjectinformation layer. In the foregoing case, it is preferable further toprovide tracking polarity inverting means for switching the polarity ofthe tracking control means in synchronization with the operation of thefocus jumping circuit. According to the foregoing preferred structure,even in a case of a recording medium having the first information layerand the second information layer having information pits formed in theopposite directions between the two layers when viewed from a positionupon which the light beam is made incident, the light beam caninstantaneously be moved onto the information pit of the subjectinformation layer.

[0042] It is preferable that the apparatus for optically recording andreproducing information according to the present invention has astructure such that the focus control means includes a first focuscontrol means for performing control to make the focal point of thelight beam to coincide with a position near the information layer,second focus control means permitted to be operated in a range smallerthan that of the first focus control means, and focus switch means forperforming switching to the second focus control means after theoperation of the first focus control means has been completed. Accordingto the foregoing preferred structure, servo operation of eachinformation layer can be performed stably while maintaining the focuspulling performance similar to that obtainable from the conventionalstructure.

[0043] It is preferable that the apparatus for optically recording andreproducing information according to the present invention has astructure such that the focus control means has at least two types ofoperation conditions to be adaptable to each of the information layers,the focus control means being arranged to select one of the operationconditions in accordance with a result of selection performed by thelayer selection means. According to the foregoing preferred structure,focus deviation can be corrected so that information can be recorded orreproduced satisfactorily.

[0044] It is preferable that the apparatus for optically recording andreproducing information according to the present invention has astructure such that the tracking control means has at least two types ofoperation conditions to be adaptable to each of the information layers,the tracking control means being arranged to select one of the operationconditions in accordance with a result of selection performed by thelayer selection means. According to the foregoing preferred structure,tracking deviation can be corrected so that information can be recordedor reproduced satisfactorily.

[0045] It is preferable that the apparatus for optically recording andreproducing information according to the present invention furthercomprises a photodetector for performing a focus control including afirst divisional light receiving surface for receiving a portion oflight reflected by the recording medium and a second divisional lightreceiving surface for receiving the reflected light on the same plane onwhich the first divisional light receiving surface is positioned, thesecond divisional light receiving surface receiving the reflected lighton the outside of the first divisional light receiving surface.According to the foregoing preferred structure, when information isreproduced from a recording medium having a multilayer structure, therange in which the focus is pulled can be enlarged by switching thefocus detection region between the moment when the focus is pulled andthe moment when the servo operation is performed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0046]FIG. 1 is a cross sectional view showing the structure of anoptical information recording medium having two information layersaccording to the present invention;

[0047]FIG. 2 is a perspective view showing the structure of an opticalinformation recording medium having two reproduction only informationlayers according to the present invention;

[0048]FIG. 3 is a perspective view showing the structure of an opticalinformation recording medium having a reproduction only informationlayer and a recording and reproducing information layer according to thepresent invention;

[0049]FIG. 4 is a perspective view showing the structure of an opticalinformation recording medium having two recording and reproducinginformation layers according to the present invention;

[0050]FIG. 5 is a cross sectional view showing the structure of anoptical information recording medium having four information layersaccording to the present invention;

[0051]FIG. 6 is a schematic cross sectional view showing an apparatusfor manufacturing the optical information recording medium according tothe present invention;

[0052]FIG. 7 is a first partial cross sectional view showing theapparatus for manufacturing the optical information recording mediumaccording to the present invention;

[0053]FIG. 8 is a second partial cross sectional view showing theapparatus for manufacturing the optical information recording mediumaccording to the present invention;

[0054]FIG. 9 shows the sequence for manufacturing the opticalinformation recording medium having two information layers according tothe present invention;

[0055]FIG. 10 shows the sequence for manufacturing the opticalinformation recording medium having four information layers according tothe present invention;

[0056]FIG. 11 is a block diagram showing the structure of an apparatusfor optically recording and reproducing information according to thepresent invention;

[0057]FIG. 12 is a schematic view showing the structure of an opticalpickup of the apparatus for optically recording and reproducinginformation according to the present invention;

[0058]FIG. 13 is a schematic view showing the structure of a focuscontrol section of the apparatus for optically recording and reproducinginformation according to the present invention;

[0059]FIG. 14 is a graph showing the waveforms of focus error signalsobtainable from the two information layers of the optical informationrecording medium according to the present invention;

[0060]FIG. 15 is a schematic view showing a photodetector of theapparatus for optically recording and reproducing information;

[0061]FIG. 16 is a schematic view showing the structure of a focuscontrol section of the apparatus for optically recording and reproducinginformation according to the present invention;

[0062]FIG. 17 is a schematic view showing the structure of a trackingcontrol section of the apparatus for optically recording and reproducinginformation according to the present invention;

[0063]FIG. 18 is a cross sectional view showing another example of theoptical information recording medium having a reproduction onlyinformation layer and a recording and reproducing information layeraccording to the present invention;

[0064]FIG. 19 is a cross sectional view showing still another example ofthe optical information recording medium having a reproduction onlyinformation layer and a recording and reproducing information layeraccording to the present invention;

[0065]FIG. 20 is cross sectional view showing another example of theoptical information recording medium having two recording andreproducing information layers according to the present invention; and

[0066]FIG. 21 shows the process for manufacturing a conventional opticalinformation recording medium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0067] An optical information recording medium and an opticalinformation recording and reproducing apparatus according to the presentinvention will now be described with reference to the drawings.

[0068]FIG. 1 is a cross sectional view showing an embodiment of theoptical information recording medium according to the present invention.As shown in FIG. 1, guide grooves for tracking, sample pits orinformation pits corresponding to information signals are formed oneither surface of a first substrate 1 having thickness d1. On theforegoing surface of the first substrate 1, there is formed a firstinformation layer 2 formed by a thin film which reflects a portion of alight beam 7 made incident on the first substrate 1 and permitspenetration of a portion of the light beam 7, the thin film 2 havingthickness d2. On a surface of a second substrate 3 having thickness d3,there are formed guide grooves or information pits corresponding toinformation signals. On the surface of the second substrate 3, there isformed a second information layer 4 formed by a thin film having areflectance higher than that of the first information layer 2 and havingthickness d4. Between the first information layer 2 and the secondinformation layer 4, there is formed a transparent separation layer 5for positioning the first information layer 2 and the second informationlayer 4 apart from each other for a predetermined distance d5. Thus, anoptical information recording medium having a double-layer structure isconstituted.

[0069] It is preferable that the first substrate 1 and the secondsubstrate 3 are formed vertically symmetrically with respect to theseparation layer 5 as much as possible That is, it is preferable thatthe materials and the thicknesses (d1 and d3) are made to besubstantially the same and the two substrates are different from eachother in only the pattern of the information pits on the surfacesthereof and the structures of the first and second information layers 2and 4.

[0070] If the-optical information recording medium is constituted asdescribed above, a vertically symmetric structure with respect to theseparation layer 5 can be formed. Therefore, even if a stress or thelike is generated in the substrate due to change in the temperature orthe like during the manufacturing process, any generated distortion canbe compensated. Even if unusual deformation factors act on the twosubstrates due to change in the environment temperature or humidity,deformation and warp can be inhibited. As a result, a structure durableagainst change in the environment can be obtained. Therefore, even ifthe first and second substrates 1 and 3 are made of a resin, deformationof the manufactured recording medium causing an error to take place inreproducing signals can be prevented.

[0071] Information signals can be reproduced from the opticalinformation recording medium according to this embodiment in such amanner that the two information layers (the first and second informationlayers 2 and 4) are irradiated with the light beam 7 from a position onthe outside of the first substrate 1, and then change in the quantitiesof reflected light beams is detected so that the information signalsrecorded on the first and second information layers 2 and 4 arereproduced. To enable the information signals to be reproduced, thelight beam 7 for irradiation must be converged efficiently onto each ofthe first and second information layers 2 and 4.

[0072] Therefore, the first information layer 2 must have apredetermined reflectance to enable the information signals formed onthe first information layer 2 to be reproduced as the change in thereflected light- Moreover, to allow the light beam 7 for irradiation andhaving a predetermined intensity to reach the second information layer4, the first information layer 2 must have a predeterminedtransmissivity. Although the second information layer 4 is not requiredto a specific transmissivity, the second information layer 4 must havethe highest possible reflectance to enlarge the change in the quantityof reflected light to provide the information signal. That is, when theinformation signals of the second information layer 4 are reproduced,the light beam 7 must, penetrate the first information layer 2 twotimes. Therefore, this embodiment has the structure such that thereflectance of the second information layer 4 is set to be higher thanthat of the first information layer 2.

[0073] As a material of the first and second substrates 1 and 3, it ispreferable that a material be employed which does not considerablyabsorb light in a wavelength region of the light beam 7 for theirradiation and which has great strength. Accordingly, the material ofthe first and second substrates 1 and 3 is a resin, such as apolycarbonate resin, polymethylmethacrylate (PMMA) resin or the like, orglass.

[0074] In the case where the resin is used as the material of the firstand second substrates 1 and 3, the substrate can be formed by heatingthe resin to the molten state and filling it into a mold havinginformation pits or a guiding groove on one side thereof. In the casewhere glass is used as the material of the first and second substrates 1and 3, a method is employed in which information in the form ofinformation pits is formed on the surface of a flat glass plate by, forexample, etching. As an alternative to this, a method (aPhoto-Polymerization method) is employed in which an ultraviolet curingresin is applied to the surface of the flat glass plate, followed bypressing a die having a surface comprising information pits to forminformation in the form of information pits. However, the methods offorming the first and second substrates 1 and 3 are not limited to theforegoing methods. Any method may be employed as long as the employedmethod is able to give a predetermined optical characteristic to thesubstrates. Since the foregoing methods are known methods employed in aprocess for manufacturing a usual optical disk, such as a compact disk,further description thereof is omitted.

[0075] The patterns of convex and concave portions to be formed on thesurfaces-of the first and second substrates 1 and 3 are different fromeach other depending upon whether the function of the information layersis an exclusively reproducing type or a recording and-reproducing type.In the case where the information layers are arranged to be exclusivelyreproducing, the convex and concave portions are in the form ofinformation pit columns constituted by a pattern formed on the surfacesof the substrates and modulated in response to the information signals.In the case where the information layers are arranged to be recordingand reproducing, the convex and concave portions are in the form ofguide grooves consisting of continuous convex and concave portions forperforming tracking control of the light beam or wobble pits adaptableto a tracking method, called a “sample servo method”.

[0076] It is preferable that the first and second substrates 1 and 3have substantially the same size, and are formed by using the samematerial in the same process. In particular, if a resin material isemployed as the material of the substrates and if the substrates areformed by the injection molding method, the substrates can becomedeformed, such as warped, after being allowed to stand for a long timedepending upon the molding conditions or the like. Moreover, change inthe environment temperature or humidity causes the substrates to beconsiderably deformed. In the case where the substrates are formed bythe Photo-Polymerization method, similar deformation takes place, thoughthe degree of the deformation is not critical as is experienced with theinjection molding method. While considering the foregoingcharacteristics, this embodiment has the structure such that the firstand second substrates 1 and 3 are formed in a similar process and thetwo substrates are adhesive-bonded to each other with the separationlayer 5. As a result of the employment of the vertically symmetricstructure with respect to the separation layer 5, stress and distortionof the substrates can be inhibited. Thus, an optical informationrecording medium having a satisfactory durability against change in theenvironment can be obtained.

[0077] The thicknesses of the two substrates, with which substantiallythe same mechanical strengths can be attained, depend upon thetemperature of the environment in which the recording medium exists orthe material of the substrates. To obtain an allowance of the differencein the thicknesses of the two substrates, an optical informationrecording medium having the following structure was manufactured: apolycarbonate resin having a thickness of 0.6 mm was employed to formthe first substrate 1. Then, an Au film having a thickness of 10 nm wasformed on the first substrate 1 so that a first information layer 2 wasformed. The thickness of the second substrate 3 made of thepolycarbonate resin was changed from 0.3 mm to 1.2 mm, and then an Aufilm having a thickness of 100 nm was formed so that a secondinformation layer 4 was formed. Moreover, a separation layer 5 wasformed by an acrylic-type ultraviolet curing resin layer having anaverage thickness of 40 μm. The separation layer 5 was used toadhesive-bond the first information layer 2 and the second informationlayer 4 to each other. The quantity of deformation of thethus-manufactured optical information recording medium was measured,thus resulting in the following values being obtained. That is, if thethickness of the second substrate 3 is 0.6 mm ±30% or smaller, allowingto stand in a room temperature environment, the temperature of which was30° C. and the relative humidity (RH) of which was 80% for 1000 hours,resulted in a quantity of warp of the recording medium of 0.4 mm orsmaller. In this case, a stable servo operation was possible. Even ifthe optical information recording medium was allowed to stand in a moresevere environment, the temperature of which was 80° C. and the relativehumidity of which was 80% for 1000 hours, warp of the recording mediumcould be inhibited in a case where the thickness of the second substrate3 is 0.6 mm ±20% or smaller.

[0078] The information layer is classified into two types exemplified bya reproduction-only type and recording and reproducing type. Therecording medium according to this embodiment has two informationlayers. Therefore, the structure of the recording medium may be, in thesequential order as the first information layer and the secondinformation layer, any of the following four types, that is, (A){reproduction only}-{reproduction only}, (B) {reproductiononly}-{recording and reproducing}, (C) {reproducing andreproducing}-{reproduction only}, and (D) {recording andreproducing}-{recording and reproducing}.

[0079] The reproduction only information layer is formed by a thin filmformed on the surface of the substrate having the foregoing informationpits formed thereon, the thin film having a predetermined reflectancewith respect to the light beam. In the foregoing case, a material may beselected from a group consisting of metal, such as Au, Al, Cu or theiralloys; an oxide, such as SiO₂, SiO, TiO₂, MgO or GeO₂; a nitride, suchas Si₃N₄ or BN; a dielectric material of a sulfide, such as ZnS or PbS;their mixtures; and a multilayer structure of the foregoing oxide,nitride and the sulfide. By using the foregoing material, informationlayers having a predetermined reflectance with respect to the light beamhaving a specific wavelength can be obtained.

[0080] In the case where the first information layer 2 is a reproductiononly information layer, the first information layer 2 must have apredetermined transmissivity with which the first information layer 2reflects the light beam 7 made incident upon from a position on theoutside of the first substrate 1 and allows the light beam having apredetermined intensity to reach the second information layer 4. In thecase where the same material is used to form the first information layer2 and the second information layer 4, the foregoing object can beobtained by making the thickness of the first information layer 2 to bethinner than that of the second information layer 4. In the case where ametal is employed to form the information layer, the metal is formedinto a thin film having a thickness of 5 nm to 40 nm. To maintain bothreflectance and the transmissivity of the first information layer 2 athigh levels, it is preferable that absorption of light by theinformation layer is the lowest possible level. In the foregoing case,the first information layer 2 may be a dielectric material or an organicmaterial capable of realizing a high refractive index and a lowcoefficient of absorption. Moreover, a layer formed by stacking thedielectric material and the organic material enables an informationlayer that does not considerably absorb light to be obtained.

[0081] In the case where the second information layer 4 is thereproduction only information layer, the transmissivity is not requiredto be considered. It is preferable that the reflectance be the highestpossible reflectance. In the case where the metal is employed to formthe second information layer 4, a metal formed into a thin film having athickness of 40 nm to 200 nm is employed.

[0082] The recording and reproducing type information layer comprises athin film formed on the substrate having the guide grooves or the samplepits formed thereon, the thin film having the optical characteristicwhich is changed when it absorbs the light beam for the irradiation andthe state of the change being identified with the light beam. As therecording layer for use as the information layer, a material to beemployed may be selected from the group which consists of a phase-changematerial, the reflectance of which is changed due to light for theirradiation because the state of the thin film is changed; amagneto-optic material in the form of the thin film, the direction ofthe magnetization of which is changed and the change of which can bedetected as a Kerr effect; an organic material, such as a coloringmatter, having the spectral reflection factor which is changed; and aphotochromic material.

[0083] The phase-change material, the phase of which is changed betweenamorphous and crystal, may be selected from a group consisting of achalcogen material, such as SbTe, InTe, GeTeSn, GeSbTe, SbSe, TeSeSb,SnTeSe, InSe, TeGeSnO, TeGeSnAu or TeGeSnSb type material; and an oxidematerial, such as Te-TeO₂, Te-TeO₂-Au or Te-TeO₂-Pd type material.

[0084] The phase-change material, the phase of which is changed betweencrystal and crystal, may be a metal compound, such as an AgZn compoundor an InSb compound.

[0085] As the magneto-optic material, MnBi, TbFe or TbFeCo type materialmay be employed.

[0086] As the organic coloring matter, a leuco dye, such astriphenylmethane or the like may be employed. The photochromic materialmay be spiropyran, fulgide or azo type material.

[0087] Note that the recording-enabled information layer is, in view ofits function, classified into a write-once type information layer towhich information can be recorded only one time and a rewriting typeinformation layer on which recorded information can be rewritten. In thecase of the write-once type information layer, only one layer made ofthe phase-change material layer or the organic coloring matter layer isrequired to be, as the information layer, formed on the substrate.Another method may be employed in which a double-layer structureconsisting of a light absorbing thin film layer and a metal layer isemployed to prepare alloys by irradiation with light.

[0088] Although the information layer may be constituted by only arecording layer, it is preferable that a plural-layer-structureincluding at least two layers be employed in order to cause the materialforming the information layer to be reversibly changed and the opticalchange in the recorded signal to be enhanced. The double-layer structuremay be a structure including a dielectric material layer/a recordinglayer, a structure including a recording layer/a reflecting layer or astructure including a reflecting layer/a recording layer (in theforegoing sequential order when viewed from a position upon which thelight beam 7 is made incident). A triple-layer structure may be, whenviewed from the substrate, a structure including a dielectric materiallayer/a recording layer/a dielectric material layer or a structureincluding a dielectric material layer/a recording layer/a reflectinglayer. A quadruple layer structure may be a structure including, whenviewed from a position upon which the light beam 7 is made incident, adielectric material layer/a recording layer/a dielectric materiallayer/a reflecting layer. A quintuple structure may including a firstreflecting layer/a dielectric material layer/a recording layer/adielectric material layer/a second reflecting layer when viewed from thesubstrate. By forming the recording layer and the dielectric materiallayer to be in contact with each other, deterioration in the thin filmwhen recording is performed repeatedly can be prevented. Moreover,optical change in recorded information can be set to a great degree.

[0089] The dielectric material layer may be made of a material selectedfrom a group consisting of an oxide, such as SiO₂, SiO, TiO₂, MgO orGeO₂; a nitride, such as Si₃N₄ or BN; a sulfide, such as ZnS or PbS; andtheir mixtures.

[0090] The reflecting layer may be made of any material exemplified whenthe reproduction only information layer has been described.

[0091] In order to maintain a sufficiently large quantity of light onthe second information layer 4, it is preferable that the separationlayer 5 be made of a material which does not considerably absorb lighthaving the wavelength region of the light beam 7, in particular, lightwhich has passed through the first information layer 2. Therefore, theseparation layer 5 may be made of a transparent adhesive agent, glasssimilar to that of the substrate or a resin material. In the case wherethe first and second substrates 1 and 3 are made of the resin material,it is preferable that a similar-type resin material be employed tomaintain mechanical reliability after adhesive-bonding. To shorten thetime required to complete the adhesive-bonding process, it is preferablethat a ultraviolet curing resin be employed.

[0092] The distance d5 of the separation layer 5 must be at least longerthan the focal depth determined by the numerical aperture (NA) of anobjective lens 6 and the wavelength (λ) of the light beam 7 in order toinhibit influence of the crosstalk from another information layer wheneither information layer is being reproduced. If the intensity of thelight convergent point is 80% or greater with respect to the centralintensity (100%) when a light beam is coverged in a stigmatic case, thefocal depth Δ z allows approximation with the following Equation (1):

Δ z=λ/{2 (NA))²}  (1)

[0093] In an exemplary case where λ=780 nm and the NA=0.55, then Δ z=1.3μm. Therefor, a region within ±1.3 μm is included in the focal depth. Inthe case where the foregoing optical system is employed, it ispreferable that the thickness d5 of the separation layer 5 be set to avalue larger than 2.6 μm.

[0094] The influences of recording marks included in the light beampassing through the first information layer 2 when the light beam 7 isfocused to the second information layer 4, act as crosstalk taking placewhen the second information layer 4 is reproduced. Therefore, in orderto reproduce the signals stably, it is preferable that the thickness d5of the separation layer 5 be at least longer than the focal depth, morepreferably five times the focal depth. A usual optical disk of areproduction-only type has information pits formed on the opticalrecording medium at a pitch shorter than the focal depth. If thethickness of the separation layer is made to be five time the focaldepth, the number of the information pits on the first information layer2, which is irradiated with the light beam 7, is 25 or more, which issufficiently smaller than a usual allowance of −26 dB to prevent thecrosstalk.

[0095] To maintain the high recording density of information to beformed on the first and second information layers 2 and 4, the first andsecond information layers 2 and 4 must be formed in a range into whichthe objective lens 6 is able to converge light beams. That is, the valueof d1+d5, which is the result of addition of the thickness d5 of theseparation layer 5 to the thickness d1 of the first substrate 1, must bewithin a tolerance for the thickness of the base which is permitted bythe optical system (the objective lens 6).

[0096] Therefore, it is preferable that the thickness d5 of theseparation layer 5 be larger than the focal depth of the optical systemfor converging the light beams 7 and smaller than the tolerance for thebase permitted by the foregoing optical system. If the foregoingconditions are satisfied, information, which is not considerablyaffected by the crosstalk from an information layer other than thesubject information layer, can be reproduced when the aberration of thelight beam 7 is small. Note that the thickness d5 of the separationlayer 5 must be set to an optimum value in consideration of the yieldwhen the optical information recording mediums are mass-produced asrecording mediums as well as the optical aberration. By forming thefirst and second substrates 1 and 3, the first and second informationlayers 2 and 4 and the separation layer 5 by using the foregoingmaterials, a recording medium can be obtained which allows informationsignals to be reproduced from the first and second information layers 2and 4 when the information layers 2 and 4 are irradiated with light froma position on the outside of the first substrate 1.

[0097] A structure of a recording medium capable of stably and easilyreproducing signals from the two information layers will now bedescribed. To reproduce information signals recorded on the twoinformation layers stably and easily, it is preferable that the levelsof signals obtainable from the two information layers be similar to eachother also in view of simplifying the structure of the reproducingapparatus. The description will hereinafter be directed to a structurein which the quantities of reflected light obtainable from the flatportions of the two information layers are similar to each other whenviewed from a position upon which the light beam 7 is made incident. Inorder to facilitate the approximation, the description will be directedto the structure in which both of the first and second informationlayers 2 and 4 are reproduction only information layers. Note that anassumption is made that the influence of diffraction of transmittedlight occurring due to the information pits of the first informationlayer 2 can be ignored.

[0098] An assumption is made that the reflectance of the firstinformation layer 2 is RI, the absorption ratio of the same is A1 andthe reflectance of the second information layer 4 is R2. In theforegoing case, another assumption is made that the quantities ofreflected light from the flat portions of the two information layers arethe same when the amplitudes of signals from the two information layersare the same. The foregoing fact is equivalent to a fact that thequantity T of a light beam 7 that has been incident and has penetratedthe first information layer 2 and been reflected by the secondinformation layer 4 and then penetrated the first information layer 2again, and the reflectance R1 of the first information layer 2 are thesame. In the foregoing case, the relationships expressed by thefollowing equations hold:

R 1 ≈T   (2)

R 1 ≈(1−A 1−R 1)² ×R 2   (3)

R 1≈1−A 1+(2−R 2)⁻¹−{[1−A1+(2·R 2)⁻¹[²−(1−A 1)²}^(0.5)   (4)

[0099] R1 is made to be a maximum value when R2=1 and A1=0. In thiscase, R1 is 0.382. If R2=0.9 and A1=0.1 in a practical view point, thenR1=0.311.

[0100] The foregoing phenomenon means that, if the diffraction of lighttransmitted through the first information layer 2 is ignored and thedegrees of diffraction of light reflected by the first and secondinformation layers 2 and 4 are the same, the forming of a secondinformation layer 4 having a reflectance R2 of 90% and a firstinformation layer 2 having a reflectance R1 of 31% and an absorptioncoefficient A1 of 10% results in the amplitudes of signals to be madethe same when information signals are reproduced from the informationpits of the first and second information layers 2 and 4.

[0101] Referring to Equation (2), a practical structure of the twoinformation layers will now be described. An assumption is made herethat the range in which the reproduction amplitudes from the twoinformation layers can be considered to be equivalent is±20%. Theforegoing fact means that the difference between the right side and theleft side of Equation (2) is ±20% or smaller.

[0102] Thus, substitution of Equation (3) with the foregoingrelationship results in the relationship expressed by the followingEquation (5) being holding in a case where the reproduction amplitudefrom the first information layer 2 is smaller by 20% than that from thesecond information layer 4. On the other hand, in a case where thereproduction amplitude from the first information layer 2 is larger by20% than that from the second information layer 4, the followingEquation (6) hold:

R 1=1.2×(1−A 1−R 1)² ×R ²   (5)

R 1=0.8×(1−A 1−R 1)² ×R ²   (6)

[0103] While considering the practical characteristic of each layer inthe case where the information layers are made of metal or thedielectric material, it is preferable that the reflectance R2 of thesecond information layer 4 be within a range from 70% to 90%. Moreover,it is preferable that the refractive index A1 of the first informationlayer 2 be within a range not greater than 20%. Substituting portions ofEquations (5) and (6) with the foregoing relationships, the range of thereflectance R1 of the first information layer 2 is 21% to 42%. Tomaintain the great reproduction amplitudes from both of the first andsecond information layers 2 and 4, it is preferable that the absorptioncoefficient A1 of the first information layer 2 be small. If the valueof the absorption coefficient A1 is 10% or smaller, the range of thereflectance R1 of the first information layer 2 is 25% to 40%.

[0104] As described above, in the case where both of the first andsecond information layers 2 and 4 are reproduction only informationlayers, it is preferable that the reflectance R1 of the firstinformation layer 2 be within a range of 25% to 40% to make thereproduction amplitudes from the first and second information layers 2and 4 to be similar to each other and to maintain the large reproductionamplitude.

[0105] In the case where the second information layer 4 is a recordingand reproducing information layer, the reflectance of the informationlayer is smaller than that of the reproduction only information layer.In a case where the reflectance of the second information layer 4 is30%, use of the first information layer 2 having a reflectance of 19.5%and a coefficient of absorption of 0% makes the quantities of lightreflected toward the objective lens 6 to be the same when both of theinformation layers are irradiated with light. Although the structure ofthe recording and reproducing apparatus will be described later, theamplitude of the reproduced signal and the quantity of reflected lighthave predetermined allowances in consideration of the difference betweenrecording mediums and contamination of the surfaces of the recordingmediums. Therefore, if the accuracy of the reproducing circuit and thestability of the apparatus are considered, it is preferable that thedifference between the amplitudes of the two reproducing signals besmaller than 5 times.

[0106] The present invention is characterized in that the substrateshaving convex and concave portions formed previously are adhesive-bondedto each other so that a recording medium having two information layersis obtained. Therefore, the information pits pattern to be formed on thesurface of the first substrate 1 must have signal shapes that can bereproduced through the base of the first substrate 1. Moreover, theinformation pits pattern to be formed on the surface of the secondsubstrate 3 must have signal shapes that can be reproduced whenirradiated with light through the surfaces of the information pits.Accordingly, the information pits pattern of the first and secondsubstrates 1 and 3 are formed to run in the same direction when viewedfrom a position upon which the light beam 7 is made incident. In thecase where the information layers are the reproduction only informationlayers, the direction of the information pits pattern is made to be inthe direction in which the information pits are formed to correspond tothe information signals. In the case where the information layers arethe information recording and reproducing layers, the foregoingstructure is applied to address information for administering the guidegrooves.

[0107] The structure of the recording medium will now be described indetail in which the first and second information layers 2 and 4 are areproduction only information layer and a reproduction only informationlayer categorized as (A).

[0108] In view of reducing the manufacturing cost of the substrates, itis preferable that the first and second substrates 1 and 3 bemanufactured by the same process as much as possible. The informationpits are formed on the surfaces of the substrates by a method includingof a mastering step for manufacturing a master and an injection moldingstep in which a resin material is injected into the master placed in amold to form a substrate having information pits. Since the masteringmethod is a known method which is usually employed when compact disks orCD-ROMs are manufactured, the detailed description of this method isomitted here. Briefly, a photoresist is applied to a flat glass plate,following by irradiating the surface of the photoresist with Ar laserbeams modulated in response to the information signals, followed byremoving the photoresist, and followed by plating the surface from whichthe photoresist has been removed so that a master is manufactured.

[0109] If the same mastering process is employed, the shapes of theinformation pits to be formed on the substrates, that is, therelationships of the information pits whether they are formed in theform of convex and concave portions with respect to a plane, are made tobe the same. FIG. 2(a) shows an example of an optical informationrecording medium manufactured by adhesive-bonding the substrates formedby the same process. As shown in FIG. 2(a), information pits 11 of thefirst substrate 1 have convex shapes when viewed from the position uponwhich the light beam 7 is made incident. Information pits 12 of thesecond substrate 3 have concave shapes when viewed from the positionupon which the light beam 7 is made incident. In the case where thecharacteristics of the photoresists are different from each other,information pits of the first substrate 1 have concave shapes whenviewed from the position upon which the light beam 7 is made incident.Information pits of the second substrate 3 have convex shapes whenviewed from the position upon which the light beam 7 is made incident.In any case, the directions of the information pits formed in the twoinformation layers 2 and 4 are opposite to each other when viewed fromthe position upon which the light beam 7 is made incident.

[0110] As a result of the foregoing structure, the same material andmolding process can be employed as well as the mastering process.Therefore, two types of manufacturing apparatuses having the samefunction are required to be prepared when the optical informationrecording medium is manufactured or one manufacturing apparatus isrequired to be used commonly. Therefore, the cost of the manufacturingfacilities can be reduced.

[0111] Since the recording medium having the foregoing structure compristhe pits formed in the opposite directions between the two informationlayers 2 and 4 when viewed from a position upon which the light beam 7is made incident, the tracking polarity must be switched between theinformation layers 2 and 4 when a tracking method, such as a push-pullmethod, is used to record or reproduce information. To prevent this, thedirection of the pits of the second information layer 4 is required tobe reversed to have convex shapes when viewed from the position uponwhich the light beam 7 is made incident. By manufacturing the secondinformation layer 4 with a master having the photoresist with thereversed characteristic in the mastering process or by using a secondmaster obtainable by again transferring a master obtained by theconventional method, pits formed in the opposite directions can beobtained between the two information layers 2 and 4.

[0112] The sizes of the information pits of the first and secondsubstrates 1 and 3 will now be described. The size of the pits to beformed on the second substrate 3 is classified into two types dependingupon whether the distance from the surface of the first substrate 1 uponwhich the light beam 7 is made incident to the surfaces of the first andsecond information layers 2 and 4 is within tolerance Δ Wd of thethickness of the base permitted by the optical system for converging thelight beam 7. Note that the tolerance Δ Wd of the thickness of the baseis determined by the spherical aberration of the light beam 7, thetolerance Δ Wd of the thickness of the base being generally in inverseproportion to the fourth power of the numerical aperture (NA) of theobjective lens 6 (see FIG. 1). For example, in an optical system havinga wavelength λ of 780 nm and a numerical aperture NA of 0.5, thetolerance Δ Wd of the thickness of the base is about 50 μm. Note thatthe tolerance Δ Wd of the thickness of the base depends upon the densityof the pits, that is, the intervals between the pits. If the intervalsbetween the pits are long, signals can be reproduced even if a sphericalaberration takes place. Thus, the tolerance range is enlarged.

[0113] A structure is shown in FIG. 2(a) which is employed in the casewhere both of the distances from the surface of the first substrate 1upon which the light beam 7 is made incident to the surfaces of thefirst and second information layers 2 and 4 are within the tolerance ΔWd of the thickness of the base determined by the light convergingoptical system and the pit density. A main issue in the foregoing caselies in that the information pits 11 on the first information layer 2and second information layer 4 are different from each other. The reasonfor this is that the first information layer 2 has a main reflectingsurface which is in contact with the first substrate 1, while the secondinformation layer 4 has a main reflecting surface which is the interfacebetween the second information layer 4 and the separation layer 5.

[0114] In a case where the width of each of the information pits 11 onthe first substrate 1 is W11 and the width of each of the informationpits 12 on the second substrate 3 is W12, the width of the mainreflecting surface is such that the width is the pit width W11 for thefirst information layer 2 and the same is the pit width W13 for thesecond information layer 4, the pit width W13 being the width of the pitof the interface between the second information layer 4 and theseparation layer 5. In a case where a known sputtering method isemployed to form the information layers on the substrates, the thin filmwill be formed to reach the diagonal surfaces of the information pits aswell as in a direction perpendicular to the surface of the substratethough the degree depends upon the manufacturing method. Therefore, thepit width W13 on the interface with respect to the separation layer 5 issmaller than pit width W12 of the information pits 12 on the secondsubstrate 3. To make the degrees of diffraction of light reflected bythe information pits to be the same, the pit width W12 of theinformation pits 12 on the second substrate 3 must be larger than thepit width W11 of the information pits 11 on the first substrate 1.

[0115] The pit width W12 of the information pits 12 on the secondsubstrate 3 is corrected in accordance with a result of a process ofactually forming the second information layer 4. The inventors haveperformed an experiment in which Au was used to form the secondinformation layer 4. On information pits each having a pit width W12 of0.50 μm and a depth of 90 nm on the second substrate 3, there was formedan Au layer (the second information layer 4) having a reflectance of 90%or higher and a thickness of 150 nm so that the shape of the secondinformation layer 4, which corresponds to the interface with respect tothe separation layer 5, was measured. As a result, the pit width W13 was0.3 μm and the depth was 90 nm. In a case where the second informationlayer 4 is formed under the foregoing conditions, the shapes of theinformation pit to be formed on the surface of the second substrate 3 isdetermined such that the pit width W12 is 0.70 μm in consideration ofthe change in the shape of the pit due to existence of the secondinformation layer 4. In the foregoing case, information pits each havinga pit width W11 of 0.50 μm and a depth of 90 nm are formed on the firstsubstrate 1. As described above, the information pits 12 on the secondsubstrate 3 are formed to have a size larger than that of theinformation pits on the first substrate 1 in consideration of areduction in the substantial width of the information pit due to thethickness of the second information layer 4. Note that track pitches Tp1and the pit densities of the information pits to be formed on thesurfaces of the first and second substrates 1 and 3 are the same.

[0116] In the mastering process for forming the second substrate 3, thesize of the information pit is enlarged as compared with that on thefirst substrate 1 by setting the power of a light source for exposingthe photoresist to light to be a slightly larger value. The otherprocesses for forming the first and second substrates 1 and 3 are thesame.

[0117] Although the case has been described in which the pit widths weremade to be different from each other between the first and secondsubstrates 1 and 3, the conditions for forming the information layerssometimes result in a consideration being made such that the angle ofinclination of the diagonal surface of the pits on the substrates isdifferent from the angle of inclination of the diagonal surfaces of theformed information layers. In the foregoing case, the depth of the piton the first substrate 1 and that on the second substrate 3 are made tobe different from each other, or both of the pit width and the pit depthare made to be different between the same. As a result of the foregoingstructure, the degrees of diffraction of reflected light with respect tothe incidental light beam 7 taking place between the first and secondinformation layers 2 and 4 can be made to approach each other. Thus,stable signal reproduction can be performed.

[0118]FIG. 2(b) shows a structure to be employed in a case where eitherof the two information layers is out of the tolerance of the thicknessof the base permitted by the light converging optical system. A mainissue in the foregoing case lies in that convergence of the light beam 7to an information layer which is out of the tolerance Δ Wd of thethickness of the base results in a spherical aberration being generatedand thus sufficient convergence of light spots being inhibited. Thephenomenon in which either of the two information layers is out of thetolerance

[0119] Δ Wd of the thickness of the base of the light convergenceoptical system takes place due to reduction in the tolerance Δ Wd of thethickness of the base occurring when the wavelength of the light beam 7has been shortened and the numerical aperture (NA) of the objective lenshas been enlarged for the purpose of raising the density of informationon the information layer. The foregoing phenomenon occurs in a casewhere a thin separation layer cannot easily be obtained when theseparation layer 5 is formed or in a case where the accuracy of thethickness of the separation layer 5 is unsatisfactory.

[0120] In the foregoing case, the density of pits on the surface of thesubstrate having the information layer which is out of the tolerance ΔWd of the thickness of the base is required to be lower than the densityof the pits on the substrate having the information layer which iswithin the tolerance Δ Wd of the thickness of the base. FIG. 2(b) showsan example of a structure to be employed in the case where the firstinformation layer 2 is within the tolerance Δ Wd of the thickness of thebase and the second information layer 4 is out of the tolerance Δ Wd ofthe thickness of the base. Each of information pits on the firstsubstrate 1 has a predetermined pit width W11, a track pitch of Tp1 anda pit density of Pd1. Each of information pits 14 on the secondsubstrate 3 has a predetermined pit width W14, a track pitch of Tp3 anda pit density of Pd3. While considering deterioration in the diaphragmdue to the spherical aberration of the light beam 7 on the secondinformation layer 4, the pit width W14 and the track pitch Tp3 of theinformation pits 14 on the second substrate 3 are made to be larger thanthose of the first substrate 1.

[0121] As a result of the foregoing structure, even if the secondinformation layer 4 is out of the tolerance Δ Wd of the thickness of thebase of the optical system for converging the light beam 7, a change inthe quantity of reflected light similar to that obtainable from thefirst information layer 2 can be obtained from the pit portions. Thus,stable signal reproduction can be performed.

[0122] Then, a pattern of the information pits or that of the guidegrooves on the surfaces of the substrates in the direction of the trackwill now be described. Although the information pits to be formed on thefirst and second substrates 1 and 3 may be formed into concentriccircles, it is preferable that a spiral form be employed similar to thatof the conventional optical disk in view of capability that the accuracyof the track pitch and the like can be improved as compared with thestructure having the concentric circles when the mastering process isperformed. The structure having the spiral projection and pit column isclassified into two types to meet the purpose.

[0123] A first structure is arranged such that the projection and pitcolumns on the first and second substrates 1 and 3 run in the samedirection when viewed from the light incidental portion. In theforegoing case, the light beam is moved in one direction from the insideto the outside or one direction from the outside to the insideregardless of the information layer having the information pits whichare subjected to tracking. In a case where a structure is employed inwhich the light beam is moved from the inside to the outside, areproduction method may be employed in which administration informationis detected in the internal portion of either of the information layersand an access is made to a desired information region including theportion between the information layers. Therefore, the foregoingstructure is suitable to a recording medium in which high speed accessmust be permitted.

[0124] As a method of obtaining the second substrate 3 with which theforegoing structure can be realized, the foregoing method may beemployed in which the second master to which the master has beentransferred again is used to reverse the directions of the informationpits. That is, transference of the surface of the master manufactured bythe photoresist to the second master enables columns of information pitsformed in the opposite directions and in the opposite spiral directionsto be obtained. By adhesive-bonding the second substrate 3 and the firstsubstrate 1 through the separation layer 5, a recording medium havingthe same spiral direction when viewed from a light incidental positioncan be obtained.

[0125] In a case where the process for manufacturing the second masteris omitted, the recording direction employed in the mastering process ischanged so that the first and second information layers 2 and 4 havingthe same spiral direction when viewed from the light incidental positionare obtained. That is, when the photoresist is exposed to light, thedirection in which the flat glass plate is made to be opposite to thatemployed when the master of the first substrate 1 is manufactured sothat the first and second information layers 2 and 4 having the samespiral direction when viewed from the light incidental position areobtained. Note that the information pits when viewed from the lightincidental position are made to be opposite to each other between thefirst and second information layers 2 and 4.

[0126] A second structure is arranged such that the projection and pitcolumns on the first and second substrates 1 and 3 run in oppositedirections when viewed from the light incidental position. In theforegoing case, the direction in which the light beam is moved (in adirection from the inside to the outside or a direction from the outsideto the inside) is made to be opposite in accordance with the informationlayer having the information pits which have been subjected to tracking.

[0127] The foregoing structure is effective in a case where informationcontinued for a long time is treated. An example of a structure will nowbe described in which the light beam is moved from the inside portion tothe outside portion of the information pits on the first informationlayer 2 and the light beam is moved from the outside portion to theinside portion of the information pits on the second information layer4. After the light beam has reproduced final information in the outerportion of the first information layer 2, the light beam is moved to theouter portion of the second information layer 4 (that is, an opticalpickup makes an access to the information start point of the secondinformation layer 4 while keeping the same position). The light beamcontinuously starts reproduction of information at the outer portion ofthe second information layer 4. Since the foregoing informationreproducing method does not involve the optical pickup being moved whenthe light beam is moved from a layer to another layer, time loss caneffectively be prevented during the movement of the light beam. In acase where the recording pits are in a CLV mode (a constant linevelocity mode), the position of the optical pickup is not changed and,therefore, the change in the rotational speed can effectively beprevented.

[0128] As a method of mastering the second substrate 3 which is able torealize the foregoing surface, a method may be available in which theposition at which recording of the signals starts is made to be oppositeto that employed in the exposing process when the master of the firstsubstrate 1 is manufactured. In a case where recording of information ofthe first substrate 1 starts at an inner position, a master exposed tolight from the outer portion is used. Since the recording mediummanufactured by using the thus-obtained first and second substrates 1and 3 comprises information pits on the first and second substrates 1and 3 which are formed in the opposite directions when viewed from thelight incidental position, the tracking polarity must be switchedbetween the information layers.

[0129] As another method of mastering the second substrate 3, a methodmay be employed in which exposure of the outer portion is performedsimilar to the foregoing case in a state where the direction in whichthe flat glass plate is made to be opposite to that employed when themaster of the first substrate 1 is manufactured. Since the recordingmedium manufactured by using the thus-obtained first and secondsubstrates 1 and 3 has the information pits on the first and secondsubstrates 1 and 3 which are formed in the same direction when viewedfrom the light incidental position, the necessity of switching thetracking polarity between the information layers can be eliminated.

[0130] The foregoing structures having both reproduction onlyinformation layer and a recording and reproducing information layer willnow be described, which are a reproduction only-recording andreproducing type structure (B) and a recording andreproducing-reproduction only structure (C).

[0131] When the foregoing structures (B) and (C) are subjected to acomparison, the foregoing structure (B) is advantageous in which thefirst information layer is made to be the reproduction only informationlayer and the second information layer is made to be the reproducing andrecording information layer because the light absorption into the firstinformation layer can be reduced. In the case of the foregoing structure(C) in which the first information layer is the recording andreproducing type information layer, light absorption is required torecord information on the information layer. In the foregoing case,diffraction of transmitted light due to the recording mark is generatedwhen signals are recorded on the first information layer 2. Thus, thequantity of light which is able to reach the second information layer 4is reduced.

[0132]FIG. 3 is a cross sectional view showing an example of a structureof the optical information recording medium having the foregoingstructure (B) of the reproduction only-recording and reproducing typeinformation layers. As shown in FIG. 3, information pits 38corresponding to information signals are formed on one surface of afirst substrate 31 having a thickness of d31. Moreover, on one surfaceof the first substrate 31, there is formed a first information layer 32having a predetermined transmissivity, a predetermined reflectance and athickness of d32. On one surface of a second substrate 33 having athickness of d33, there are formed tracking guide grooves 39 or samplepits. On one surface of the second substrate 33, there is formed asecond information layer 34 formed by a thin film, the opticalcharacteristic of which is changed when irradiated with the light beam 7and which has a thickness of d34. Between the first information layer 32and the second information layer 34, there is formed a transparentseparation layer 35 for positioning the first information layer 32 andthe second information layer 34 to be apart from each other for apredetermined distance d35.

[0133] The first information layer 32 has a predetermined transmissivitywith respect to the light beam 7 in order to allow light having apredetermined intensity to reach the second information layer 34. Thetemperature of a portion of the second information layer 34 that hasbeen irradiated with the light beam 7, the intensity of which has beenintensified, is raised. As a result, the optical characteristic of thesecond information layer 34 is changed so that information is recordedon the second information layer 34. Thus, the second information layer34 has a structure capable of satisfying both high coefficient ofabsorption with respect to the light beam 7 and great optical change,that is, a high efficiency in reproducing recorded signals.

[0134] Since the first information layer 32 is a reproduction onlyinformation layer, the first substrate 31 has, on the surface thereof,information pits 38 corresponding to information signals. Since thesecond information layer 34 is a recording and reproducing typeinformation layer, the second substrate 33 has, on the surface thereof,guide grooves comprising convex and concave portions for controllingtracking for locating the light beam when information is recorded orsample pits (not shown) consisting of a pair of a projection and a pitwhich are shifted in the tracking direction to correspond to thesample-servo type tracking operation. In a case where the foregoingsubstrates are in the form of a disk, it is preferable that theinformation pits, the guide grooves or the sample pits be formed into aspiral that is formed in the same direction when viewed from a positionupon which the light beam 7 is made incident.

[0135] A recording medium having the information layers of the foregoingstructure (D) consisting of the recording and reproducing informationlayer and the recording and reproducing information layer will now bedescribed with reference to FIG. 4. As shown in FIG. 4, a firstsubstrate 41 having a thickness of d41 has one surface on which trackingguide grooves 48 or sample pits are formed. The first substrate 41 hasone surface having a first information layer 42 which has apredetermined transmissivity and a predetermined reflectance, theoptical characteristic of which changes when irradiated with the lightbeam 7, and which has a thickness of d42. On one surface of the secondsubstrate 43 having a thickness of d43, there are formed tracking guidegrooves 49 or sample pits. On one surface of the second substrate 43,there is formed a second information layer 44 formed by a thin film, theoptical characteristic of which is changed when irradiated with thelight beam 7 and which has a thickness of d44. Between the firstinformation layer 42 and the second information layer 44, there isformed a transparent separation layer 45 for positioning the firstinformation layer 42 and the second information layer 44 apart from eachother for a predetermined distance d45.

[0136] Also in the foregoing case, it is effective when the foregoingstructure on the surface of the substrate is adopted to information pitsfor the reproduction only information layer. In particular, the addresspit to be formed on the surface of the substrate together with the guidegrooves or the sample pits so as to administer the recording medium isable to employ all of the methods adapted to the foregoing informationpits for only reproducing information.

[0137] The information layers for recording and reproducing informationis able to employ the information layers employed in the case of theforegoing structure (B) consisting of the reproduction only informationlayer and the recording and reproducing type information layer. In theforegoing case, the first information layer 42 must have characteristicssuch that it absorbs the light beam 7 in a predetermined quantity, itsstate is changed because its temperature is raised, the changed statecan be detected as change in the reflected light, and it permits lightto pass through in a predetermined quantity in order to enable thesecond information layer 44 to record and reproduce information.Moreover, the first information layer 42 must maintain thecharacteristic that light penetration is permitted even afterinformation has been recorded. As described above, the first informationlayer 42 must be designed so as to be formed into a thin film with whichhigh quality signals can be obtained and a required transmissivityrealized before and after information has been recorded can be obtained.

[0138] The thin film forming the first information layer 42 has, likethe phase-change material, an optical constant that is changed, thechanged state being detected as change in the reflectance. In a casewhere the second information layer 44 is irradiated with the light beamin the state where information has been recorded on the firstinformation layer 42, a portion of light, which has passed through thefirst information layer 42, is diffracted. Residual beams are convergedonto the second information layer 44. Therefore, the intensity of thelight beam 7 must be set to a high level as compared with that to beapplied to the reproduction only information layer.

[0139] In view of satisfactorily reproducing signals, the thickness d45of the separation layer 45 must be larger than the focal depth, andpreferably at least, 5 times the focal depth. Thus, the number ofrecording marks included in the light beam when the first informationlayer 42 is penetrated by the light beam is made to be 25 or more whichis the square of 5. As a result, an influence of crosstalk or the likecan be inhibited.

[0140] In the case where the first information layer 42 is formed into amagneto-optic recording type thin film, the direction of magnetizationof which is changed, transmitted light does not diffract. Since thenecessity of considering the change in the transmitted light before andafter information has been recorded can be eliminated, an advantage canbe realized. However, the first information layer 42 must absorb lightin a predetermined quantity in order to record information. Therefore,it is preferable that the quantity of light to be applied be set to belarger than that set in the case of the reproduction only informationlayer.

[0141] The optical recording medium has a characteristic that areproduction only medium and a recording and reproducing medium are ableto coexist with each other. Moreover, a so-called partial ROM disk canbe manufactured in which the same medium surface has a reproduction onlyregion formed in the inner portion thereof and a recording andreproducing region formed in the outer portion thereof.

[0142] A recording medium having four information layers will now bedescribed, which is an improvement of the recording medium having thetwo substrates having convex and concave portions on the surface thereofand adhesive-bonded to each other. This recording medium is describedwith reference to FIG. 5.

[0143] As shown in FIG. 5, information pits corresponding to informationsignals or guide grooves for controlling tracking of the light beam orsample pits are formed on one surface of a first substrate 58. Moreover,a first information layer 59 for transmitting a portion of the lightbeam 7 made incident upon the first substrate 58 and having apredetermined reflectance is formed on one surface of the firstsubstrate 58. On one surface of the second substrate 60, there areformed information pits corresponding to information signals or guidegrooves for controlling tracking of the light beam or sample pits. Onone surface of the second substrate 60, a second information layer 61 isformed which has a reflectance higher than that of the first informationlayer 59. The first information layer 59 and the second informationlayer 61 are positioned opposite to each other. Between the firstinformation layer 59 and the second information layer 61, there isformed at least a first separation layer 62. A third substrate 63 havinga similar thickness to that of the first substrate 58 has a surface onwhich information pits corresponding to information signals or guidegrooves for controlling tracking of the light beam or sample pits areformed. On one surface of the third substrate 63, there is formed athird information layer 64 which transmits a portion of the light beammade incident on the third substrate 63 and which has a predeterminedreflectance. A fourth substrate 65 having substantially the samethickness as that of the second substrate 60 has a surface on whichinformation pits corresponding to information signals or guide groovesfor controlling tracking of the light beam or sample pits are formed.Moreover, on one surface of the fourth substrate 65, there is formed afourth information layer 66 having a reflectance higher than that of thethird information layer 64. The third information layer 64 and thefourth information layer 66 are formed opposite to each other. Betweenthe third information layer 64 and the fourth information layer 66,there is formed a transparent second separation layer 67 in the form ofat least one layer, the second separation layer 67 having a thicknesssimilar to that of the first separation layer 62. The second substrate60 and the fourth substrate 65 are formed opposite to each other.Between the second substrate 60 and the fourth substrate 65 there isformed an adhesive layer 68.

[0144] Since the recording medium having the foregoing structure has avertically symmetric structure with respect to the adhesive layer 68, itcan be said that the foregoing structure is stable even if change, suchas change in the environment temperature, occurs.

[0145] A method and apparatus for manufacturing the recording mediumhaving plural information layers will now be described.

[0146] If the interval between two information layer is too short in thecase where each of the two information layers is reproduced, reflectedlight from another information layer or light which has transmittedthrough the same generates crosstalk so that an influence of change inthe amplitude of the reproduced signal or that of distortion of theservo signal occurs. If the interval between the two information layersis too long, a aberration is generated in the light converged spot ineither of the information layers. To inhibit the foregoing influences,the interval between the two information layers must be made to beconstant. In order to achieve this, a separation layer exhibitingexcellent accuracy in the thickness thereof is required. Moreover, thetwo substrates must be adhesive-bonded to each other in such a mannerthat the central positions of the information pits or sample pits or theguide grooves coincide with each other. Note that the foregoingdescription is applied only to the case where the recording medium is inthe form of a disc shape and information is recorded when the recordingmedium is rotated. In a case of a recording medium having the twoinformation layers, the tracking control must be performed whileconsidering the allowable eccentricity in the case of a recording mediumhaving one information layer and a second eccentricity occurring due todeviation in the central positions between the two information layers.The present invention is intended to prevent the second eccentricity soas to compensate the tracking servo performed by the recording medium ofthe foregoing type.

[0147] In view of the foregoing, a method of manufacturing the twosubstrates will now be described. Note that the first substrate isformed by the conventional method. That is, the first substrate isobtained by the steps of manufacturing a master in the mastering processand performing injection molding in a mold. The second substrate may bemanufactured by the same process as that for manufacturing the firstsubstrate or a method in which a second master is formed by againduplicating the master in order to reverse the information pits. Theinjection molding process may be performed by a process similar to thatemployed to manufacture the first substrate. Note that the injectionmolding machine for use in this embodiment has the master comprising thespiral or concentric information pits and guide grooves, the center ofwhich accurately coincide with the center of a central opening formingmachine for forming the central openings of the first and secondsubstrates. By using the injection molding machine of the foregoingtype, first and second substrates can be obtained in which center of thecentral opening and that of the information pits or the guide grooves donot deviate considerably.

[0148] An adhesive-bonding apparatus for adhesive-bonding the first andsecond substrates with a separation layer having a predeterminedthickness will now be described. FIG. 6 is a schematic cross sectionalview of the adhesive-bonding apparatus. As shown in FIG. 6, theadhesive-bonding apparatus comprises an upper-portion support section 61for supporting the second substrate 3; a lower-portion support section62 including a light source 81 for supporting the first substrate 1 andhardening the separation layer; an elevation section 63 for elevatingthe upper-portion support section 61; a resin-applying nozzle 64 forapplying a resin material 80 for forming the separation layer to thefirst substrate 1; and a base 65 for supporting the overall system.

[0149] The upper-portion support section 61 comprises a substratesupport section 66 a which is in contact with the flat surface of thesecond substrate 3 to secure the second substrate 3; an upper basesection 66 b for establishing the connection between the substratesupport section 66 a and the elevation section 63; an upper shaft 84downwards projecting over the central portion of the substrate supportsection 66 a and having a tapered recess at a leading end thereof tocorrect the positional relationship of the lower-portion support section62; and a first inner-portion guide section 68 having a tapered portionfor adjusting the position of the second substrate 3. Around the uppershaft 84, a spring 69 for downwards pressing the first inner-portionguide section 68 with predetermined force is disposed. On a region ofthe substrate support section 66 a which is in contact with the secondsubstrate 3, there is formed a suction opening 71 for securing thesecond substrate 3 by vacuum action, with air being discharged to anexternal pump through an evacuation port 70 formed in the substratesupport section 66 a.

[0150] On the other hand, the lower-portion support section 62 comprisesa base support section 72 for supporting the first substrate 1; alight-source box 73 for securing the base support section 72 to the base65 and accommodating a light source 81; a lower shaft section 74 havinga tapered projection at the leading end thereof to face the recess ofthe upper shaft 84; and a second inner-portion guide section 75 foradjusting the position of the first substrate 1 by the tapered portionthereof. Around the lower shaft section 74, there is disposed a spring46 for upwardly pressing the second inner-portion guide section 75 withpredetermined force.

[0151] The light source 81 hardens a resin material 80 for forming theseparation layer and is disposed in the bottom portion of thelight-source box 73 and immediately below the base support section 72.Therefore, the base support section 72 is made of a material whichpermits light emitted by the light source 81 to pass through, forexample, glass or a resin. Moreover, a portion of the base supportsection 72, which is in contact with the first substrate 1, has anadsorption opening 78 for securing the first substrate 1 by vacuumadsorption so that air is discharged to an external pump through anevacuation opening 77 formed in the light-source box 73. In a portion ofthe light-source box 73 outward from the first substrate 1 and oppositeto the substrate support section 66 a, there is disposed a spacer 79 formaintaining the thickness d5 of the separation layer.

[0152] The resin-applying nozzle 64 extrudes resin material 80 suppliedfrom an external resin reservoir tank through its leading end thereof soas to apply the resin material 80 to the upper surface of the firstsubstrate 1. The leading end of the resin-applying nozzle 64 is arrangedto move on a circle, the radius of which is about ⅔ of the radius of thefirst substrate 1 relative to the central axis of the first substrate 1.Note that the resin-applying nozzle 64 is removed from the region on thelower-portion support section 62 if it does not apply the resin material80 to the upper surface of the first substrate 1.

[0153]FIG. 7 shows a structure of the apparatus according to thisembodiment for obtaining a separation layer having a predeterminedthickness. FIG. 7 is a partial cross sectional view showing the state ofthe adhesive-bonding apparatus shown in FIG. 6 in which theupper-portion support section 61 has been moved downwards and the firstsubstrate 1 and the second substrate 3 have been adhesive-bonded to eachother through the separation layer 5. To obtain the thickness d5 of theseparation layer 5, a spacer 79 having a thickness of d79 is disposedadjacent to the outer ends of the first and second substrates 1 and 3.The thickness d79 of the spacer 79 must satisfy the following equation:

d 79=d 1+d 3+d 5   (7)

[0154] where d1 is the thickness of the first substrate 1 and d3 is thethickness of the second substrate 3.

[0155] To provide the separation layer 5 having the thickness of d5 atthe inner portion of the first and second substrates 1 and 3, thelengths d67 and d74 of the upper shaft 84 and the lower shaft section 74must satisfy the following equation, assuming that the thickness of thesubstrate support section 66 a is d65 and the thickness of thelight-source box 73 is d73:

d 67+d 74=d 65+d 73+d 1+d 3+d 5   (8)

[0156] By improving the accuracy of each section, a separation layer 5in which irregularity in the thickness is inhibited from the innerportion to the outer portion can be obtained.

[0157] Referring to FIGS. 6 and 8, a structure for performing anadhesive-bonding operation with which eccentricity is inhibited betweenthe first and second information layers will now be described. Thecenters of the upper-portion support section 61 and the lower-portionsupport section 62 are adjusted by a tapered section 82 of the recess inthe upper shaft 84 and a tapered section 83 of the recess in the lowershaft 74. When the upper-portion support section 61 has been moveddownwards by the elevation section 63, the tapered section 82 of therecess in the upper shaft 84 and the tapered section 83 of the recess inthe lower shaft 74 correct the centers of the upper-portion supportsection 61 and the lower-portion support section 62. When the horizontalplanar portions of the two shafts 67 and 74 are brought into contactwith each other at the lowermost position, the deviation of the centersof the upper-portion support section 61 and the lower-portion supportsection 62 is on the order of several μm or less, which is determined bythe machining accuracy of the two shafts 67 and 74.

[0158] To keep a constant positional relationship between the first andsecond substrates 1 and 3 and the upper shaft 84 and the lower shaftsection 74, the first and second inner-portion guide sections 68 and 75are formed, which are in contact with the columnar portions of the twoshafts 84 and 74 and which have the same central axis as those of theshafts 84 and 74. The first and second inner-portion guide sections 68and 75 are formed into tapered shapes having leading ends, the diametersD68 and D75 of which respectively are smaller than the diameters D3 andD1 of the openings in substrates 3 and 1, and other ends, the diametersD69 and D76 of which respectively are larger than the diameters D3 andD1 of the substrates 3 and 1. Each of the first and second inner-portionguide sections 68 and 75 is able to move in the vertical direction ofthe upper shaft 84 and the lower shaft section 74. The springs 69 and 76disposed around the two shafts 84 and 74 press the second substrate 3downwards and pushes the first substrate 1 upwardly. As described above,the central openings of the first and second substrates 1 and 3 havingthe diameters D1 and D3 are formed with excellent positional accuracy bythe injection molding machine. The central openings of the substrates 3and 1 are received by the tapered sections of the first and secondinner-portion guide sections 68 and 75. The substrates 3 and 1 areadsorbed through the adsorption openings 71 and 78 formed in thesubstrate support sections 66 a and 72. In a state where the taperedsections of the first and second inner-portion guide sections 68 and 75are in contact with the central openings of the substrates 3 and 1, thesubstrates 3 and 1 are secured to the surface of the substrate supportsections 66 a and 72. As a result, the central axis of the informationlayer of the second substrate 3 and the central axis of theupper-portion support section 61 coincide with each other with anaccuracy that is essentially the mechanical accuracy. Similarly, thecentral axis of the information layer of the first substrate 1 and thecentral axis of the lower-portion support section 62 coincide with eachother with an accuracy that is essencially the mechanical accuracy. Bymoving the upper-portion support section 61 downwards in the foregoingstate, the tapered section 82 of the recess in the upper shaft 84 andthe tapered section 83 of the recess in the lower shaft 74 make thecenter axis of the information layer of the first substrate 1 and thecenter axis of the information layer of the second substrate 3 coincidewith each other.

[0159] By using the adhesive-bonding apparatus having the foregoingstructure, a recording medium can be obtained in which deviation of thecircular arc of the information pits, sample pits or guide groovesformed on the surfaces of the two information layers can be inhibited.

[0160] A method of manufacturing the optical information recordingmedium having the two information layers in such a manner that theadhesive-bonding apparatus shown in FIG. 6 is used will now be describedwith reference to sequence charts shown in FIG. 9.

[0161] Initially, as shown in FIG. 9(a), a first information layer 2 forpermitting a penetration of a portion of light beams and having apredetermined reflectance is formed, by sputtering or an evaporationmethod, on a first substrate 1 having on its surface, information pitscorresponding to information signals, or guide grooves for controllingtracking of the light beam or sample pits. As shown in FIG. 9(b), asecond information layer 4 having a reflectance higher than that of thefirst information layer 2 is formed, by sputtering or an evaporationmethod, on a second substrate 3 having on its surface, information pitscorresponding to information signals, or guide grooves for controllingtracking of the light beam or sample pits, and a thickness substantiallythe same as that of the first substrate 1. Then, the first substrate 1is secured to a base support section 72. Then, a resin-applying nozzle64 is used to apply a photosetting resin material 80 to the uppersurface of the first information layer 2, as shown in FIG. 9(c). Then,the second substrate 3 is placed on a substrate support section 66 a,and then an elevation section 63 is operated to move an upper-portionsupport section 61 downwards to come into contact with a spacer 79.Then, the second substrate 3 and the first substrate 1 areadhesive-bonded to each other with a separation layer 5 at an interval,which is the thicknesses d5 of the separation layer 5 (see FIG. 9(d)).Then, as shown in FIG. 9(e), the outer surface of the first substrate 1is irradiated with light beams 73 emitted from a light source 81 so thatthe resin material 80 is hardened so that the separation layer 5 isformed. As a result of the foregoing process, a recording medium havingthe two information layers 2 and 4 can be obtained.

[0162] By employing the foregoing method, the process for removing themaster is not required to obtain a recording medium having a doublelayer structure such that substrates having information pit surfacesformed previously are simply adhesive-bonded to each other. As a result,the manufacturing yield can be improved.

[0163] Although the structure has been described in which the resinmaterial 80 is applied to the upper surface of the first informationlayer 2, the resin material 80 may be applied to the upper surface ofthe second information layer 4.

[0164] A method of manufacturing a light recording medium having fourinformation layers in such a manner that adhesive-bonding apparatusshown in FIG. 6 is used will now be described with reference to thesequence charts shown in FIG. 10.

[0165] As shown in FIG. 10(a), initially, a first information layer 59for permitting a penetration of a portion of light beams and having apredetermined reflectance is formed, by sputtering or an evaporationmethod, on a first substrate 59 having on its surface, information pitscorresponding to information signals, or guide grooves for controllingtracking of the light beam or sample pits.

[0166] As shown in FIG. 10(b), a second information layer 61 having areflectance higher than that of the first information layer 59 is formedon a second substrate 60 having on its surface, information pitscorresponding to information signals, or guide grooves for controllingtracking of the light beam or sample bits, and a thickness substantiallythe same as that of the first substrate 1.

[0167] Then, the first substrate 58 is secured to a base support section72. Then, a resin-applying nozzle 64 is used to apply a photosettingresin material 80 to the upper surface of the first information layer59, as shown in FIG. 10(c). Then, the second substrate 60 is placed on asubstrate support section 66 a of the adhesive-bonding apparatus, andthen an elevation section 63 is operated to move an upper-portionsupport section 61 downwards to come into contact with a spacer 79.Then, the second substrate 60 and the first substrate 58 areadhesive-bonded to each other with a separation layer 62 at an interval,which is the thicknesses d62 of the separation layer 62. Then, the outersurface of the first substrate 58 is irradiated with light beams 102emitted from a light source 81 so that the resin material 80 is hardenedso that the separation layer 62 is formed (see FIG. 10(d)). As a resultof the foregoing process, a first recording medium 101 having twoinformation layers on one side thereof can be obtained.

[0168] The foregoing method is different from the process shown in FIG.9 in which the first and second substrates 1 and 3 having similarthicknesses are adhesive-bonded to each other, in that the thickness ofthe second substrate 60 to be adhesive-bonded with the first substrate58 is not limited in this embodiment. In the foregoing case, thethickness of the spacer 79 and the lengths of the upper shaft 84 and thelower shaft 74 must be changed.

[0169] As a result of a process (see FIGS. 10(e) to 10(h)) similar tothat shown in FIGS. 10(a) to 10(d), a second medium 103 having twolayers formed on one surface thereof is obtained by adhesive-bonding thethird substrate 63 and the fourth substrate 65. Referring to FIGS. 10(e)to 10(h), reference numeral 64 represents a third information layer, 66represents a fourth information layer, 67 represents a second separationlayer, and 104 represents a light beam emitted from the light source 81.In the foregoing case, it is preferable that the thickness of the firstsubstrate 58 and that of the third substrate 63 be substantially thesame. Moreover, it is preferable that the thickness of the secondsubstrate 60 and that of the fourth substrate 65 be substantially thesame. The second and fourth substrates 60 and 65 may be formed by amethod other than the method employed to form the first and thirdsubstrates 58 and 63. To maintain the thin substrate serving as therecording medium, for example, a Photo-Polymerization method may beemployed, permitting the substrate to be made thinner.

[0170] Then, an adhesive-bonding apparatus similar to that shown in FIG.6 is used to adhesive-bond the first medium 101 having two layers formedon one surface thereof and the second medium 103 having two layersformed on one surface thereof to each other. Initially, as shown in FIG.10(i), a photosetting resin 105 is applied to the upper surface of thesecond substrate 60 of the first medium 101 having two layers formed onone surface thereof. Then, as shown in FIG. 10(j), the second substrate60 and the fourth substrate 65 are adhesive-bonded to each other,followed by irradiating the outer surface of the first substrate 58 withthe light beams 106 emitted from the light source 81 so that the resinmaterial 105 is set to form an adhesive layer 68. As shown in FIG.10(j), the lengths of the spacer 79, the upper shaft 84 and the lowershaft 74 must be changed to correspond to the overall length of therecording medium, the lengths of the first and second separation layers62 and 67 and the lengths of the first to fourth information layers 59,61, 64 and 66. When the exposure process shown in FIG. 10(j) isperformed, the light beams 106 emitted from the light source 81penetrate the first and second information layers 59 and 61, and thenreach the resin material 105. Therefore, the quantity of the light beams106 must be enlarged as compared with the exposure processes, forexample, as shown in FIG. 9(e), 10(d) or 10(h).

[0171] As the resin material 105 for use when the first and secondmediums 101 and 103 each having two layers formed on one surface thereofare adhesive-bonded to each other, a resin that absorbs light beams maybe employed. A resin other than the photosetting resin may be employed.For example, a heat setting resin, a hot melt adhesive agent or anotheradhesive agent may be employed. Therefore, the irradiation with lightbeams may be omitted from the process shown in FIG. 10(j).

[0172] By employing the foregoing method, the process for removing themaster is not required. By sequentially adhesive-bonding substrates eachhaving the surface comprising the information pits formed previously, arecording medium having a four-layer structure can be obtained. As aresult, the manufacturing yield can be improved. The recording mediumhaving the four-layer structure can be obtained by repeating three timesthe adhesive-bonding process required to obtain the recording mediumhaving the double-layer structure. That is, the recording medium havingthe four-layer structure can be realized by using basically the samemanufacturing apparatus. Thus, it can be realized by a method similar tothat required to obtain the recording medium having the double-layerstructure.

[0173] A recording and reproducing apparatus for recording andreproducing on the optical information recording medium according to thepresent invention, as manufactured by the foregoing method, will now bedescribed with reference to FIG. 11. As Oshown in FIG. 11, the recordingand reproducing apparatus according to this embodiment comprises anoptical disk 111 that is basically an optical information recordingmedium having a plurality of information layers; a spindle motor 112 forrotating the optical disk; an optical pickup section 113 for converginglight beams, such as laser beams, emitted from a light source 121; andfive circuit systems for controlling the spindle motor 112 and theoptical pickup section 113. A first circuit system is a light modulatingsystem 114 for operating the light source 121 of the optical pickupsection 113. A second circuit system is a control system 115 forcontrolling the operation of the light beams to cause light beamsemitted from the optical pickup section 113 to be converged onto theoptical disk 111 and tracking for causing the light beams to follow theinformation pits or the guide grooves. A third circuit system is asignal reproduction system 116 for reading information signals formed onthe optical disk 111. At least one of the foregoing three circuitsystems has two or more types of condition setting functions to set anoptimum condition to each of the information layers. A fourth circuitsystem is a layer selection system 117 for switching the condition ofthe three circuit systems in accordance with the information layer towhich the light beams are to be detected. A fifth circuit system is asystem control system 118 for controlling the timing of the four circuitsystems.

[0174] The present invention has a structure such that the layerselection system 117 is used to reproduce recorded information to selectthe optimum condition of the foregoing circuit systems to enableinformation to be recorded onto a plurality of information layers whileinhibiting errors in information to be reproduced from the pluralinformation layers.

[0175] When the information signals are reproduced from the optical disk111, the system control system 118 controls the rotation control section119 to rotate the spindle motor 112 so as to rotate the optical disk 111at constant speed. A control signal indicating the reproduction state issupplied to a laser drive section 120 so that the electric currentflowing to the light source 121 is controlled in such a manner that theintensity of light beams to be emitted from the optical pickup section113 is set at a reproduction power value as instructed by the systemcontrol system 118. The light beams emitted from the light source 121pass through the optical system of the optical pickup section 113 and anobjective lens 122 disposed at the rear portion so that the light beamsare made to be converged beams with which the optical disk 111 isirradiated.

[0176] The light beams reflected by the optical disk 111 again passthrough the objective lens 122 and the optical system in the opticalpickup section 113 so as to be made incident upon a photodetector 123having a light receiving surface which is divided into sections. Thephotodetector 123 photoelectrically converts the incidental light beamsto transmit a signal having the voltage corresponding to the change inthe quantity of light on each of the light receiving surfaces to thesignal reproduction system 116. The signal transmitted from thephotodetector 123 is amplified by a pre-amplifier 124 so that lowfrequency components in the signal are used to control the position ofthe light beam.

[0177] A focus control section 126 uses a portion of the signaltransmitted from each of the light receiving surfaces of thephotodetector 123 to obtain a focus error signal and operate a voicecoil 125 by using the focus error signal. As a result, the objectivelens 122 is controlled to move slightly in the perpendicular directionwith respect to the surface of the optical disk 111 so that the lightbeams are converged onto the surface of the information layer of theoptical disk 111. The system control system 118 transmits, to a layerselection system 117, a layer selection signal for appointing aninformation layer to be focused in response to control signal S03. Thelayer selection system 117 switches the operations of the lightmodulating system 114, the control system 115 and the signalreproduction system 116 in accordance with the information layer. As aresult, signals in any information layer on the optical disk 111 can bereproduced.

[0178] A layer identification section 132 demodulates a layeridentification signal from the signal transmitted from a binary-codingsection 130 to identify the information layer that is being focused. Ifthe information layer that is being focused is not the subjectinformation layer, a focus jumping circuit 133 sequentially shifts thefocusing position among the information layers. The focus jumpingcircuit 133 superimposes the pulse voltage for instantaneously moving avoice coil 125 in the perpendicular direction with respect to theoptical disk 111 on the output signal from the focus control section126. As a result, the light beam can be converged onto the subjectinformation layer.

[0179] A tracking control section 127 obtains a tracking control signalfrom a combination of the other output signals from the photodetector123 in such a manner that the light beam follows the information pits orthe guide grooves, and then slightly moves the voice coil 125 in adirection of the radii of the optical disk-111. When the informationlayer of a reproduction only type is reproduced, a polarity inverter 128switches the tracking polarity among the information layers inaccordance with an instruction issued from the layer selection system117 so that a phase difference method or a 3-beam method is employed toperform tracking in such a manner that the light beams reproduce theinformation pits of the information layer. In a case where theinformation layer of a recording and reproducing type is reproduced, thepolarity inverter 128 switches the tracking polarity or the trackingmethod among the information layers in accordance with an instructionissued from the layer selection system 117. Thus, the information pitsof the information layer of the recording and reproducing type arereproduced by tracking by a push-pull method in a case where theinformation layer has guide grooves or by a sample servo method in acase where the information layer is composed of wobble pits. Byswitching the tracking method according to the type of the informationlayer, the recording density of both of the information layers can beraised.

[0180] A tracking jumping circuit 129 superimposes the pulse voltage forinstantaneously moving the voice coil 125 in the direction of the radialof the optical disk 111 on the output signal from the tracking controlsection 127. As a result, the light beam can be moved onto the surfaceof the subject track.

[0181] In accordance with the output from the tracking control section127, the polarity inverter 128 inverts the polarity thereof inaccordance with the direction of the information pits formed on theinformation layer, and whether the light beam is allowed to follow theland or groove of the guide groove.

[0182] In the case where the recording medium has been manufactured byusing the first and second substrates 1 and 3 obtained by the samemastering process as shown in FIG. 2(a), the directions of the pits areinverted between the first and second information layers 2 and 4 whenviewed from a position upon which the light beam 7 is made incident. Inthe case of a double-layer medium of the foregoing type, the focusingposition is moved between the first and second information layers 2 and4 by the focus jumping circuit 133. Simultaneously, the polarityinverter 128 switches the tracking polarity between the first and secondinformation layers 2 and 4. As a result, the light beam can be movedinstantaneously onto the information pit of the subject informationlayer. Therefore, the foregoing method is able to reduce the timerequired to make an access between the information layers wheninformation is reproduced.

[0183] The binary-coding section 130 of the signal reproduction system116 uses the high frequency components of the signal supplied from thepre-amplifier 124 to make a comparison between the level of theforegoing signal and a reference level so as to convert the signal intoa binary-coded signal. Then, the decoder 131 decodes the binary-codedsignal in accordance with a predetermined signal format. As a result,the information signal is demodulated from the recording mark formed onthe optical disk 111. Then, demodulation information S02 is transmittedto an external unit in accordance with an instruction issued from thesystem control system 118.

[0184] If necessary, conditions for reproducing or recording theinformation layer formed in a specific region on the optical disk 111are demodulated by the layer identification section 132. The layeridentification section 132 also has a function capable of demodulatingthe shape of the information layer and the like as well as identifyingthe information layer. It is preferable that the foregoing informationbe recorded during the process for manufacturing the recording medium.The contents of the information include identification information foridentifying whether the information layer is a reproduction onlyinformation layer or a recording and reproducing information layer orinformation for correcting the differences in the characteristics amongthe information layers. That is, the contents are information about theoptimum condition for irradiating each information layer with light, theoptimum condition for performing the focus control or the trackingcontrol and the optimum condition when the reproduced signal isdemodulated.

[0185] In a case where information is recorded on a plurality ofinformation layers, initially the system control system 118 causes thelight modulating system 114 to receive recording information S01composed of information to be recorded at a predetermined timing. Thelight modulating system 114 initially causes an encoder 134 to convertthe recording signal into a recording signal having a predeterminedformat, and then causes the laser drive section 120 to modulate theintensity of light to be emitted from the light source 121 in accordancewith the condition of a waveform setter 135, which divides a pulse orsets a change in the intensity. Light having the modulated intensity isabsorbed into the recording layer on the optical disk 111. As a result,the reproducing mark can be formed on the recording layer on the opticaldisk 111 so that information is recorded.

[0186] Note that the waveform setter 135 has recording patterns whichare optimum for recording on the respective information layers andchanges the output therefrom in synchronization with the output from thelayer selection system 117. The laser drive section 120 modulates theintensity of the light to be emitted from the light source 121 inaccordance with the modulated waveforms corresponding to the respectiveinformation layers.

[0187] By employing the foregoing structure, the information signals canbe reproduced from the plural information layers under optimumconditions. Moreover, the information signals can be recorded onto theplural information layers under optimum conditions and recordedinformation can be reproduced.

[0188] The specific operations of the components of the recording andreproducing apparatus will now be described in detail.

[0189]FIG. 12 shows the structure of the optical pickup section. In thisembodiment, a knife edge method is employed as the focusing method and apush-pull method is employed as the tracking method.

[0190] As shown in FIG. 12, light emitted from the light source 121passes through a collimator lens 140 so as to be formed into parallelbeams, followed by being reflected by a beam splitter 141. Then, thereflected beams pass through a λ/4 plate 142 and the objective lens 122so that the optical disk 111 is irradiated with the light beams. Lightreflected by the optical disk 111 passes through the objective lens 122,the λ/4 plate 142 and the beam splitter 141, and then passes through alens 143, and then a portion of the light beams is reflected and is madeincident upon the photodetector 145 having a plurality of lightreceiving surfaces for performing the tracking operation. An output fromeach of the light receiving surfaces of the photodetector 145 isamplified by the pre-amplifier 124 so that a tracking error signal isobtained from a difference signal.

[0191] On the other hand, light which is not reflected by the mirror 144is made incident upon the photodetector 146 having a plurality of lightreceiving surfaces for performing the focusing operation. An output fromeach of the light receiving surfaces of the photodetector 146 isamplified by the pre-amplifier 124 so that a focus error signal isobtained from the difference signal. Referring to FIG. 12, referencenumeral 113 represents the optical pickup and 120 represents the laserdrive section.

[0192]FIG. 13 shows a portion of a focus control section for performingthe focus control in accordance with the output from the photodetector.Although a usual knife edge method employs a photodetector having alight receiving surface divided into two sections, this embodiment hasan arrangement such that the photodetector 146 has a light receivingsurface divided into at least four sections, as shown in FIG. 13. Thereason for this is that if a photodetector having the light receivingsurface divided into two sections is used, then an operation ofobtaining a servo signal from a subject information layer encounters theproblem of the servo signal being distorted because a portion of lightreflected by other information layers is made incident upon thephotodetector. Although reduction in the area of the light receivingportion of the photodetector enables the distortion of the servo signalto be inhibited, another problem arises in that a range for pullingfocusing is excessively limited.

[0193] Accordingly, this embodiment has a structure such that the lightreceiving surface of the photodetector 146 is divided into at least foursections. Moreover, a method is employed in which the focus detectionregion is switched between a focus pulling stage and the servo operationstage. As shown in FIG. 13, the photodetector 146 has the lightreceiving surface divided into light receiving surfaces 146 a, 146 b,146 c and 146 d. An output from each of the light receiving surfaces 146a, 146 b, 146 c and 146 d of the photodetector 146 is amplified by eachof amplifiers 147 a, 147 b, 147 c and 147 d so that two types of focuserror signals 148 s and 149 s are obtained by difference amplifiers 148and 149. Then, a switching unit 150 selects either of the focus errorsignal 148 s or 149 s. The selected focus error signal 148 s (or 149 s)passes through a focus operation circuit 151 and the focus jumpingcircuit (see FIG. 11), and then operates the optical pickup section 113(see FIG. 11).

[0194] The focus error signal will now be described with reference toFIG. 14 in both of a case where the photodetector for the focusingoperation is divided into two sections and a case where the same isdivided into four sections. The axis of abscissa stands forfocus-directional positions in which the positions of the twoinformation layers are indicated by L1 and L2. FIG. 14(a) shows a casewhere the light receiving surface is divided into two sections and thelight receiving surface is large. FIG. 14(b) shows a case where thelight receiving surface is divided into two sections and the lightreceiving surface is small. FIG. 14(c) shows a case where the lightreceiving surface is divided into four sections and the outer lightreceiving surfaces 146 a and 146 d are used. FIG. 14(d) shows a casewhere the light receiving surface is divided into four sections and theinner light receiving surfaces 146 b and 146 c are used. In the caseshown in FIG. 14(a) where the light receiving surface is divided intotwo sections and a photodetector 151 having a large light receivingsurface is employed, and in a case where a focal point exists near focusbeam F1 from an information layer, reflected light F2 from anotherinformation layer is made incident upon a portion of the light receivingsurface. Therefore, the focus error signal is distorted and, therefore,a focal-point positional error dF occurs. In the case shown in FIG.14(b) where the photodetector 152 is used which has the light receivingsurface divided into two sections and having a small light receivingsurface area, receipt of leaked light beam from another informationlayer can be prevented. Moreover, S-figure curves of the two focus errorsignals appear at the positions corresponding to the positions L1 and L2of the information layers so that the servo operation is enabled.However, the realized focus pull-in range M2 is smaller than pull-inrange Ml in the case shown in FIG. 14(a) in which the light receivingsurface has a large area. Thus, the operation becomes unstable if therecording medium is warped or has an irregularity on its surface.

[0195] To overcome the foregoing problems, this embodiment employs thephotodetector 146 having a light receiving surface divided into foursections. Note that the light receiving surface is formed in such amanner that the reflected beam F1 from either information layer ispositioned at substantially the central position of a division linebetween the light receiving surfaces 146 a and 146 b and the reflectedbeam F2 from another information layer is positioned at substantiallythe central position of a division line between the light receivingsurfaces 146 c and 146 d. In a case where the distance between thecenters of the reflected beams F1 and F2 on the light receiving surfaceis Lf and the size of each of the spots of the reflected beams F1 and F2on the light receiving surface is Ld, width 146 w of the outer lightreceiving surfaces 146 a and 146 d is set to be larger than Lf andsmaller than Lf+Ld. FIG. 14(c) shows a focus error signal in a casewhere the light receiving surface is divided into four sections and theouter light receiving surfaces 146a and 146 b are used. Since the lightreceiving surfaces are apart from the reflected light beams F1 and F2from the two information layers, an S-figure curve appears, similar tothe case where one information layer is present. If a servo operation isperformed in response to the foregoing signal, the focusing position islocated between the position L1 of one of the information layers and theposition L2 of the other information layer. In the foregoing case, alarge range M3 can be obtained as the range in which the focusing signalis found. FIG. 14(d) shows a focus error signal in a case where thelight receiving surface is divided into four sections and inner lightreceiving surfaces 146 b and 146 c are used. The focus error signal issimilar to that of the case shown in FIG. 14(b) in which the lightreceiving surface is divided into two sections and the light receivingsurface has a small area.

[0196] The photodetector 146 having the light receiving surface dividedinto four sections according to this embodiment is able to realize alarge focus pull-in range and stable focusing with respect to the twoinformation layers by switching the focus error signal in the case shownin FIG. 14C and the focus error signal shown in FIG. 14(d).

[0197] When focusing is pulled, the switching unit 150 selects adifference signal 149 s from the outer light receiving surfaces 146 aand 146 d of the photodetector 146 so that the focus operating circuit151 starts the focusing operation. In the foregoing state, the focusingpoint is positioned between the two information layers.

[0198] When completion of the operation for pulling focusing has beenconfirmed by the focus operating circuit 151, the focus operatingcircuit 151 transmits a focus operation completion signal 151 s to theswitching unit 150. In response to the focus operation completion signal151 s, the switching unit 150 selects the difference signal 148 s whichis any of outputs from the inner light receiving surfaces 146 b and 146c of the photodetector 146 so that either of the information layers issubjected to focusing. Then, the tracking operation is performed in apredetermined region to determine whether or not the subject informationlayer has been subjected to focusing. If an information layer which isnot the intended subject has been subjected to focusing, the focusjumping circuit 133 moves the focusing position to the subjectinformation layer. During the foregoing process, the switching unit 150does not perform the switching operation.

[0199] Although the description has been directed to a structure whichemploys the knife edge method as the focusing method, the focusingmethod is not limited to this. For example, an astigmatism method may beemployed. If the astigmatism method is employed, a cylindrical lens isdisposed at the position of the mirror 144 shown in FIG. 12 and aphotodetector 154 having a light receiving surface divided into 8sections as shown in FIG. 15 is disposed near the photodetector 146. Inthe case where the astigmatism method is employed, outputs from thelight receiving surfaces 154 a, 154 b, 154 c and 154 d in the vicinityof the photodetector 146 are used similar to the knife edge method.After the focus pull-in operation has been completed, outputs from thelight receiving surfaces 154 e, 154 f, 154 g and 154 h in the centralportion of the photodetector 146 are used so that a focus error signalis obtained.

[0200] By employing the foregoing structure, a stable servo operationfor each of plural information layers can be performed while maintainingthe focus pull-in performance similar to that obtainable from theconventional structure.

[0201] The quality of a recording medium having plural informationlayers depends upon the irregularity of the shapes of the informationpits or guide grooves. The quality of the recording and reproducingapparatus depends upon the distortion of the intensity distribution ofthe light beams or dispersion of the sensitivity of the photodetector orthe like. Therefore, error voltage is generated in the focus errorsignal or the tracking error signal due to interference between theinformation layers or change in the thickness of the separation layerwhen the servo operation is performed.

[0202] In order to correct an error in the focus control signal or thetracking control signal, the focus control section or the trackingcontrol section is offset-adjusted in synchronization with the settingof the layer selection system 117 (see FIG. 11). For example, a fineoffset is added to the focus control signal so that focus deviationgenerated between the layers is corrected. Also a fine offset is addedto the tracking control signal so that tracking deviation is corrected.Thus, an optimum light convergent state can be realized in each of theinformation layers.

[0203]FIG. 16 shows the focus control section in detail. As shown inFIG. 16, a focus error signal 160 s is produced by a focus errordetection circuit 160, obtained from a signal in the output signal 124 sfrom the pre-amplifier 124 (see FIG. 12) and relating to the focuscontrol. Thus, a focus control signal 126 s can be obtained by a focusoperation circuit 162 through an offset compensation circuit 161. Thefocus control signal 126 s is transmitted to the optical pickup section113 (see FIG. 11) so that the voice coil 125 (see FIG. 11) is operatedand the focus control is performed.

[0204] The offset compensation circuit 161 has a structure capable ofsetting a plurality of offset levels in response to a signal suppliedfrom the outside. An offset setting unit for setting the offset to besupplied to the offset compensation circuit 161 comprises an offsetsetting unit 163 for setting an offset when focusing of the firstinformation layer 2 has been performed; and an offset setting unit 164for setting an offset when focusing of the second information layer 4has been performed. An offset selector 165 responds to an output 117sfrom the layer selection system 117 (see FIG. 11) to transmit the offsetvalue of either of the offset setting unit 163 or the offset settingunit 164.

[0205] On the other hand, the focus operation circuit 162 receives asignal 161s transmitted from the offset compensation circuit 161 andtransmits a focus control signal 126s for making the signal 161 s to bezero so as to operate the voice coil 125. The gain setting unit forsetting the gain of a circuit when the focusing operation is performedcomprises a gain setting unit 166 for setting the gain in the case ofthe first information layer 2; and a gain setting unit 167 for settingthe gain in the case of the second information layer 4. A gain selectionunit 168 responds to an output 117 s from the layer selection system 117to transmit a signal from the gain setting unit 166 or the gain settingunit 167. By employing the foregoing structure, an optimum focusingstate can be set with respect to the two information layers.

[0206] As for the tracking control, the setting of an optimum statebetween the information layers enables reproduction or recording andreproducing to be performed more satisfactorily. FIG. 17 shows thetracking control section in detail. As shown in FIG. 17, a trackingerror signal 170 s can be obtained by a tracking error detection circuit170 from a signal relating to the tracking control in an output signal124 s from the preamplifier 124. A tracking control signal 127 s can beobtained by a tracking operation circuit 172 through an offsetcompensation circuit 171. The tracking control signal 127 s passesthrough the polarity inverter 128 (see FIG. 11) and is supplied to theoptical pickup section 113. Thus, the voice coil 125 is operated so thatthe tracking control is performed.

[0207] The offset compensation circuit 171 is structured to be capableof setting a plurality of offset levels in response to a signal suppliedfrom the outside. The offset setting unit for setting offset to besupplied to the offset compensation circuit 171 comprises an offsetsetting unit 173 for setting the offset when focusing of the firstinformation layer 2 has been performed; and an offset setting unit 174for setting the offset when focusing of the second information layer 4has been performed. The offset selector 175 responds to an output 117 sfrom the layer selection system 117 (see FIG. 11) to transmit an offsetvalue of the offset setting unit 173 or the offset setting unit 174.

[0208] On the other hand, the tracking operation circuit 172 receives asignal 171 s transmitted from the offset compensation circuit 171 andtransmits a tracking control signal 127 s with which the signal 171 s ismade to be zero so that the voice coil 125 is operated. The gain settingunit for setting the gain when the tracking operation is performedcomprises a gain setting unit 176 for setting the gain in the case ofthe first information layer 2; and a gain setting unit 177 for settingthe gain in the case of the second information layer 4. A gain selector178 corresponds to an output 117 s from the layer selection system 117to transmit a signal from the gain setting unit 176 or the gain settingunit 177. By employing the foregoing structure, an optimum trackingstate with respect to the two information layers can be set.

[0209] Although this embodiment has a structure such that theinformation pits formed on the first substrate 1 are in the convex formwhen viewed from a position at which the light beam is made incident,the structure is not limited to this. The information pits may be formedas concave when viewed from a position at which the light beam is madeincident. In the foregoing case, the directions of the information pitson the second substrate 3 are inverted so that a recording medium havinga similar effect to that obtainable from this embodiment is obtained.

[0210] Specific structures of the information recording medium will nowbe described.

EXAMPLE 1

[0211] A method of manufacturing the optical information recordingmedium shown in FIG. 1 and an operation of recording and reproducing theoptical information recording medium will now be described.

[0212] Polycarbonate resin was employed to form the first and secondsubstrates 1 and 3, and a mold having a surface comprising informationpits was used to perform injection molding so that the first and secondsubstrates 1 and 3 were manufactured. The first substrate 1 had adiameter of 120 mm and a thickness of 1.2 mm and comprised, on thesurface thereof, information pits arranged such that the shortest pitlength was 0.83 μm, the pit depth was 100 nm and the track pitch was 1.6μm. The information pits consisted of pit columns formed to conform tothe EFM code. On the first substrate 1, there was formed an Au layerhaving a thickness of 10 nm by a sputtering method so that the firstinformation layer 2 was formed.

[0213] The second substrate 3 had the same diameter and thickness asthose of the first substrate 1, the second substrate 3 havinginformation pits in the same form as those of the first substrate 1. Tomake the directions of the spirals of the first substrate 1 and thesecond substrate 3 to be the same when viewed from a position upon whichthe light beam 7 was made incident after adhesive-bonding, the directionof the spiral of the projection and pit column when viewed from thesurfaces of the information pits of the second substrate 3 was invertedwith respect to that of the first substrate 1. An Au film was formed tohave a thickness of 100 nm by a sputtering method so that the secondinformation layer 4 was formed. The information pits of the first andsecond substrates 1 and 3 were formed into concave when viewed from aposition at which the pits exist.

[0214] The first substrate 1 was secured to the substrate supportsection 72 of the adhesive-bonding apparatus shown in FIG. 6, followedby using the resin applying nozzle 64 to apply the acrylic typeultraviolet curing resin material 80 to the upper surface of the firstinformation layer 2. The second substrate 3 was placed on the substratesupport section 66 a of the adhesive-bonding apparatus. Then, theupper-portion support section 61 was moved downwards by the elevationsection 63 to bring the same into contact with the spacer 79. Whilepressing the second substrate 3 from an upper position with a load of 5kg, it was irradiated with light emitted from the light source (theultraviolet lamp) 81. Thus, the resin material 80 was hardened so thatthe separation layer 5 having a thickness of d5 was formed between thefirst information layer 2 and the second information layer 4.

[0215] Before the adhesive-bonding was performed, the thickness of theinner portion, intermediate portion and the outer portion of eachsubstrate was previously measured to calculate the differences fromthose after adhesive-bonding. Thus, the thickness d5 of the separationlayer 5 was obtained. As a result, the mean value of the thickness ofthe separation layer 5 was 65 μm with an accuracy of ±8 μm or less ateach measurement position. The reflectance of the first informationlayer 2 at a wavelength of 780 nm was 27.5% and that of the secondinformation layer 4 at a wavelength of 780 nm was 91.6%. The level ofeccentricity between the information layers was 40 μm.

[0216] Information was reproduced from the foregoing recording mediumwith an optical system having a light source for emitting light having awavelength of 780 nm and an objective lens adaptable to an optimumthickness of the base of 1.2 mm and having a numerical aperture (NA) of0.5. A knife edge method was employed to perform focusing, while apush-pull method was employed to perform tracking. Focusing wasperformed by using the photodetector 146 shown in FIG. 13 and having alight receiving surface divided into four sections in such a manner thatthe light receiving surface of the photodetector 146 for obtaining afocus error signal was switched between the pull-in operation and theservo operation. Reproducing light had power of 1 mW when the signal wasreproduced. As a result, it was confirmed that stable focusingoperations were performed with respect to the first and secondinformation layers 2 and 4 and focus jumping was stably performed. Notethat the polarity of the tracking signal was switched between theinformation layers. An excellent eye pattern was observed in theobtained reproduced signal from both of the first and second informationlayers 2 and 4. Jitters of both signals were measured, thus resulting inexcellent values to be obtained such that the standard deviation withrespect to the width of the detection window was 8.4% in the case of thefirst information layer 2 and 8.7% in the case of the second informationlayer 4.

[0217] Then, the obtained recording medium was allowed to stand in a hotand wet environment, the temperature of which was 80=and the relativehumidity was 80%, for 100 hours, followed by performing a similarexperiment. Then, the signal was similarly evaluated. As a result,change in the shape was inhibited, information could stably bereproduced, and an excellent result was obtained in measuring jitterswithout considerable change.

[0218] As a result, the method according to the present invention is aneffective method for manufacturing a recording medium having a pluralityof information layers.

EXAMPLE 2

[0219] A structure of a recording medium capable of forming informationmore densely will now be described. Similarly to Example 1,polycarbonate resin was employed to form the first and second substrates1 and 3, and a mold having a surface comprising information pits wasused to perform injection molding so that the first and secondsubstrates 1 and 3 were manufactured. The first substrate 1 had athickness of 0.58 mm and comprised, on the surface thereof, informationpits arranged such that the shortest pit length was 0.5 μm, the pitdepth was 90 nm and the track pitch was 0.8 μm. On the first substrate1, there was formed an Au layer having a thickness of 11 nm by asputtering method so that the first information layer 2 was formed.

[0220] The second substrate 3 had the same thickness as that of thefirst substrate 1, the second substrate 3 having information pits in thesame form as those of the first substrate 1. To make the directions ofthe spirals of the first substrate 1 and the second substrate 3 to bethe same when viewed from a position near the light source afteradhesive-bonding, the direction of the spiral of the projection and pitcolumn when viewed from the surfaces of the information pits of thesecond substrate 3 was inverted with respect to that of the firstsubstrate 1. An Au film was formed to have a thickness of 100 nm by asputtering method so that the second information layer 4 was formed. Tomake the shape of the pit on the main reflecting surface after thesecond information layer 4 had been formed to be similar to that of thefirst substrate 1, the length of the shortest pit of the pits to beformed on the surface of the second substrate 3 was made to be 0.6 μm.However, the pitch of the pits and the track pitch were the same ofthose of the first substrate 1.

[0221] The first substrate 1 was secured to the substrate supportsection 72 of the adhesive-bonding apparatus shown in FIG. 6, followedby using the resin applying nozzle 64 to apply the acrylic typeultraviolet curing resin material 80 to the upper surface of the firstinformation layer 2. The second substrate 3 was placed on the substratesupport section 66 a of the adhesive-bonding apparatus. Then, theupper-portion support section 61 was moved downwards by the elevationsection 63 to bring the same into contact with the spacer 79. Whilepressing the second substrate 3 from an upper position with a load of 8kg, it was irradiated with light emitted from the light source (theultraviolet lamp) 81. Thus, the resin material 80 was hardened so thatthe separation layer 5 having a thickness of d5 was formed between thefirst information layer 2 and the second information layer 4.

[0222] Before the adhesive-bonding was performed, the thickness of theinner portion, intermediate portion and the outer portion of eachsubstrate are previously measured to calculate the differences fromthose after adhesive-bonding. Thus, the thickness d5 of the separationlayer 5 was obtained. As a result, the mean value of the thickness ofthe separation layer 5 was 52 μm with an accuracy of +5 μm or less ateach measurement position. The reflectance of the first informationlayer 2 at a wavelength of 680 nm was 28.2% and that of the secondinformation layer 4 at a wavelength of 680 nm was 89.6%. The level ofeccentricity between the information layers was 35 μm.

[0223] Information was reproduced from the foregoing recording mediumwith an optical system having a light source for emitting light having awavelength of 680 nm and an objective lens adaptable to an optimumthickness of the base of 0.6 mm and having a numerical aperture (NA) of0.6. A similar servo method to that employed in Example 1 was employed.As a result, it was confirmed that stable focusing operations wereperformed with respect to the first and second information layers 2 and4 and focus jumping between the information layers was stably performed.Note that the polarity of the tracking signal was switched between theinformation layers. An excellent eye pattern was observed in theobtained reproduced signal from both of the first and second informationlayers 2 and 4. Jitters of the both signals were measured, thusresulting in excellent values to be obtained such that the standarddeviation with respect to the width of the detection window was 7.6% inthe case of the first information layer 2 and 8.0% in the case of thesecond information layer 4.

[0224] Then, the obtained recording medium was allowed to stand in a hotand wet environment, the temperature of which was 80° C. and therelative humidity was 80%, for 100 hours, followed by performing asimilar experiment. Then, the signal was similarly evaluated. As aresult, change in the shape was inhibited, information could stably bereproduced, and an excellent result was obtained in measuring jitterswithout considerable change.

EXAMPLE 3

[0225] An example of a recording medium having four information layersas shown in FIG. 5 will now be described. Similarly to Example 1,polycarbonate resin was employed to form the first to fourth substrates58, 60, 63 and 65, and a mold having a surface comprising informationpits was used to perform injection molding so that the first to fourthsubstrates 58, 60, 63 and 65 were manufactured. Each of the first andthird substrates had a thickness of 0.58 mm and comprised, on thesurface thereof, information pits arranged such that the shortest pitlength was 0.5 μm, the pit depth was 90 nm and the track pitch was 0.8μm. On each of the first and third substrates 58 and 63, there wasformed an Au layer having a thickness of 11 nm by a sputtering method sothat the first and third information layers 59 and 64 were formed.

[0226] Each of the second and fourth substrates 60 and 65 had athickness of 0.4 mm, which was smaller than that of each of the firstand third substrates 58 and 63, in order to reduce the overall thicknessof the recording medium after adhesive-bonding, each of the second andfourth substrates 60 and 65 having information pits having the sameshapes as those of the first and second substrates 58 and 63 on thesurface thereof. To make the directions of the spirals of the first andsecond substrates 58 and 63 and the second and fourth substrates 60 and65 to be the same when viewed from a position near the light sourceafter adhesive-bonding, the direction of the spiral of the informationpit column of the second and fourth substrates 60 and 65 was invertedwith respect to that of the first and second substrates 58 and 63. Onthe second and fourth substrates 60 and 65, an Au film was formed tohave a thickness of 100 nm by a sputtering method so that the second andfourth information layers 61 and 66 were formed. To make the shapes ofthe pits on the main reflecting surfaces after the second and fourthinformation layers 61 and 66 had been formed to be similar to those ofthe first and third substrates 58 and 63, the shortest length of the pitamong the pits to be formed on the surfaces of the second and fourthsubstrates 60 and 65 was made to be 0.6 μm.

[0227] The first substrate 58 was secured to the substrate supportsection 72 of the adhesive-bonding apparatus shown in FIG. 6. The resinapplying nozzle 64 was used to apply the acrylic ultraviolet curingresin material 80 to the upper surface of the first information layer59. The second substrate 60 was placed to the substrate support portion66 a of the adhesive-bonding apparatus. Then, the elevation section 63was operated to move the upper-portion support section 61 downwards tobe brought into contact with the spacer 79. While applying a load of 8kg from an upper position to the second substrate 60, it was irradiatedwith light emitted from the light source (the ultraviolet lamp) 81 sothat the resin material 80 was hardened. As a result, the firstseparation layer 62 was formed between the first information layer 59and the second information layer 61. The third substrate 63 was securedto the substrate support section 72 of the adhesive-bonding apparatus.Then, the resin applying nozzle 64 was used to apply the acrylic typeultraviolet curing resin material 80 to the upper surface of the thirdinformation layer 64. The fourth substrate 65 was placed on thesubstrate support section 66 a of the adhesive-bonding apparatus. Then,the elevation section 63 was operated so that the upper-portion supportsection 61 was moved downwards to be brought into contact with thespacer 79. While applying a load of 8 kg to the fourth substrate 65 froman upper position, it was irradiated with light emitted from the lightsource (the ultraviolet lamp) 81 so that the resin material 80 washardened. Thus, a second separation layer 67 was formed between thethird information layer 64 and the fourth information layer 66.

[0228] Before the adhesive-bonding operation was performed, thethickness of each of the inner portion, the intermediate portion and theouter portion was measured to calculate the difference from that afterthe adhesive-bonding process. Thus, the thicknesses of the first andsecond separation layers 62 and 67 were obtained, thus resulting in thatthe mean thicknesses of the first and second separation layers 62 and 67were 50 μm and 53 μm, respectively. The accuracy at each measurementposition was within ±7 μm. In the foregoing case, the reflectance ofeach of the first and third information layers 59 and 64 at wavelengthof 680 nm was 28.5%, and that of the second and fourth informationlayers 61 and 66 at wavelength of 680 nm was 88.7%. The levels ofeccentricity between the information layers of the first and secondmediums 101 and 103 each having two layers were excellent values, thatis, 30 μm and 28 μm, respectively. Then, the first medium 101 wassecured to the substrate support section 72 of the adhesive-bondingapparatus. The resin applying nozzle 64 was used to apply the acrylictype ultraviolet curring resin material 105 to upper surface of thesecond substrate 60 of the first medium. The second medium 103 wasplaced on the substrate support section 66 a of the adhesive-bondingapparatus. Then, the elevation section 63 was operated so that theupper-portion support section 61 was moved downwards to be brought intocontact with spacer 79. While applying a load of 10 kg to the secondmedium 103 from an upper position, it was irradiated with enlarged lightbeam 106 emitted from the light source 81 so that the resin material 105was hardend. Thus the first medium 101 and the second medium 103 areadhesive-bonded to each other via adhesive layer 68.

[0229] Information was reproduced from the foregoing recording medium bythe optical system similar to that employed in Example 2 and the servomethod. It was confirmed that a stable focusing operation was performedwhen two information layers were reproduced from either of the surfaceof the first and third substrates 58 and 63. Moreover, focus jumpingbetween the information layers was stably performed. An excellent eyepattern was observed in the reproduced signal from either of theinformation layers. Jitters of each of the first to fourth informationlayers 59, 61, 64 and 66 were measured, thus resulting in excellentstandard deviations with respect to the detection window width of 7.9%,8.3%, 7.9% and 8.2%.

[0230] An experiment was performed such that the foregoing recordingmedium was allowed to stand in a hot and wet environment in which thetemperature was 80° C. and the relative humidity was 80%, for 100 houresand then the signal was similarly evaluated. As a result, change in theshape was inhibited, information could stably be reproduced, and themeasurement of jitters resulted in a satisfactory value withoutconsiderable change.

EXAMPLE 4

[0231] The specific structure of the optical information recordingmedium shown in FIG. 18 and recording and reproducing operations withthe foregoing medium will now be described.

[0232] The first substrate 31 was made of polycarbonate resin andhaving, on the surface thereof, information pits formed in accordancewith EFM modulation to correspond to information signals. The thicknessd1 of the first substrate 31 was 0.58 mm and the diameter was 120 mm.The information pits formed on the surface of the first substrate 31were arranged in such a manner that the shortest length of the pit ofthe pits formed on its surface was 0.44 μm, the pitch depth was 90 nmand the track pitch was 0.74 μm. A ZnS layer having a thickness of 40 nmwas formed on the surface of the first substrate 31 by the sputteringmethod so that the first information layer 32 was formed.

[0233] The second substrate 33 was made of polycarbonate resin and hadguide grooves for tracking light beams on the surface thereof. Thesecond substrate 33 had a thickness of 0.58 mm and a diameter of 120 mm.The pitch of the guide grooves formed on its surface was 1.48 μm, thewidth of the groove was the half of the pitch, and the depth was 70 nm.On the surface of the second substrate 33, there were formed areflecting layer 180 made of Al, a ZnS-SiO₂ dielectric-material layer181, a Ge-Sb-Te recording thin film layer 182 and a ZnS-SiO₂dielectric-material layer 183 stacked sequentially. Thus, the secondinformation layer 34 was formed.

[0234] The first substrate 31 was secured to the substrate supportsection 72 of the adhesive-bonding apparatus shown in FIG. 6. The resinapplying nozzle 64 was used to apply the ultraviolet curing type resinmaterial 80 to the upper surface of the first information layer 32. Thesecond substrate 33 was placed on the substrate support section 66 a ofthe adhesive-bonding apparatus. Then, the elevation section 63 wasoperated to downwards move the upper-portion support section 61 to bebrought into contact with the spacer 79. While applying a load to thesecond substrate 33 from an upper position, it was irradiated with lightemitted from the light source (the ultraviolet lamp) 81. Thus, the resinmaterial 80 was hardened so that the separation layer 35 was formedbetween the first information layer 32 and the second information layer34. The mean thickness of the separation layer 35 was 40 μm with anaccuracy of within ±8 μm or better at each measurement position. Notethat the thickness d1 of the first substrate 31 was 0.6 mm, which wasthe optimum thickness of the base for the objective lens 6 forconverging the light beam 7. The arrangement was determined in such amanner that the optimum point was made to be the central position of theseparation layer 35. In the foregoing case, the reflectance of the firstinformation layer 32 at a wavelength of 680 nm was 10% and thereflectance of the second information layer 34 at a wavelength of 680 nmwas 17%.

[0235] Recording and reproducing of information to and from theforegoing recording medium were evaluated using an optical systemcomprising a light source for emitting light having a wavelength of 680nm and an objective lens having numerical aperture (NA) of 0.6 adaptableto the optimum base thickness of 0.6 mm, the evaluation being performedat a linear velocity of 6 m/s. Note that the power of the reproducingbeam was 1 mW when the signal was evaluated. As a result, a stablefocusing operation was performed with respect to the first and secondinformation layers 32 and 34. Moreover, focus jumping betweeninformation layers was stably performed. As the tracking method, a phasedifference method which is suited to the reproduction of a informationpits with a narrow track pitch, was used for the first information layer32, and a push-pull method, which is suited to a guiding groove, wasused for the second information layer 34. An excellent eye pattern wasobserved in the reproduced signal from the first information layer 32.Jitters of each mark were measured, thus resulting in a standarddeviation of jitters with respect to the detection window width for thecode signal of 8.4%.

[0236] EFM signals having a shortest mark length of 0.6 μm were recordedon both land portions and groove portions of the guide grooves on thesecond information layer 34. As a result of irradiation with lightmodulated between recording power of 10 mW and deletion power of 5 mW,excellent eye pattern was observed in each case. The amplitude of thelongest mark 11T was similar to that obtainable from the firstinformation layer 32. Measured jitters resulted in excellent values of9.7% in the land portion and 9.5% in the groove portion. The foregoinginformation signals could be repeatedly rewritten. Note that theforegoing characteristics were equally observed from the inner portionto the outer portion of the substrate.

[0237] An experiment was performed such that the foregoing recordingmedium was allowed to stand in a hot and wet environment in which thetemperature was 80° C. and the relative humidity was 80%, for 100 houresand then the signal was similarly evaluated. As a result, change in theshape was inhibited, information could stably be reproduced, and themeasurement of jitters resulted in a satisfactory value withoutconsiderable change.

[0238] As a result, it can be said that the foregoing method is aneffective method of manufacturing a recording medium having a pluralityof information layers.

EXAMPLE 5

[0239] The specific structure of the optical information recordingmedium shown in FIG. 19 and operations for recording and reproducinginformation to and from the same will now be described.

[0240] The first substrate 31 was made of polycarbonate resin and had,on the surface thereof, information pits formed in accordance with EFMmodulation to correspond to information signals. The thickness d1 of thefirst substrate 31 was 0.58 mm and the diameter of the same was 120 mm.The shortest length of the pit of the information pits on the firstsubstrate 31 was 0.44 μm, the pit depth was 90 nm and the track pitchwas 0.74 a m. On the surface of the first substrate 31, there wereformed sequentially by the sputtering method, a dielectric-materiallayer 194 having a thickness of 140 nm and made of ZnS-SiO₂, adielectric layer 195 having a thickness of 30 nm and made of SiO2 and adielectric-material layer 196 having a thickness of 140 nm and made ofZnS-SiO₂. Thus, the first information layer 32 was formed.

[0241] The second substrate 33 was made of polycarbonate resin and hadguide grooves for tracking light beams. The second substrate 33 had athickness of 0.58 mm and a diameter of 120 mm. The pitch of the guidegrooves formed on the surface of the second substrate 33 was 1.1 μm andthe depth of the groove was 50 nm. The second substrate 33 had, on thesurface thereof, a reflecting layer 198 having a thickness of 50 nm andmade of Au, a ZnS-SiO₂ dielectric-material layer 198 having a thicknessof 50 nm, a Ge-Sb-Te recording thin film layer 199 having a thickness of10 nm, a ZnS-SiO₂ dielectric-material layer 200 having a thickness of 20nm and a semitransparent reflecting layer 201 having a thickness of 14nm and made of Au, stacked sequentially. Thus, the second informationlayer 34 was formed.

[0242] The first substrate 31 was secured to the substrate supportsection 72 of the adhesive-bonding apparatus shown in FIG. 6. The resinapplying nozzle 64 was used to apply the ultraviolet curing type resinmaterial 80 to the upper surface of the first information layer 32. Thesecond substrate 33 was placed on the substrate support section 66 a ofthe adhesive-bonding apparatus. Then, the elevation section 63 wasoperated to downwards move the upper-portion support section 61 to bebrought into contact with the spacer 79. While applying a load to thesecond substrate 33 from an upper position, it was irradiated with lightemitted from the light source (the ultraviolet lamp) 81. Thus, the resinmaterial 80 was hardened so that the separation layer 35 was formedbetween the first information layer 32 and the second information layer34. The mean thickness of the separation layer 35 was 43 μm with anaccuracy of within ±9 μm or better at each measurement position. Notethat the thickness of each substrate was 0.58 mm. The objective lens 6for converging the light beam 7 was adapted to an optimum base thicknessof 0.6 mm. The arrangement was determined in such a manner that theoptimum point was made to be the central position of the separationlayer 35. In the foregoing case, the reflectance of the firstinformation layer 32 at a wavelength of 680 nm was 17% and thereflectance of the second information layer 34 at a wavelength of 680 nmwas 45%.

[0243] Recording and reproducing of information to and from theforegoing recording medium were evaluated by using an optical systemcomprising a light source for emitting light having a wavelength of 680nm and an objective lens having numerical aperture (NA) of 0.6 adaptableto the optimum base thickness of 0.6 mm, the evaluation being performedat a linear velocity of 1.3 m/s. Note that the power of the reproducingbeam was 1 mW when the signal was evaluated. As a result, a stablefocusing operation was performed with respect to the first and secondinformation layers 32 and 34. Moreover, focus jumping betweeninformation layers was stably performed. As the tracking method, a phasedifference method was used for the first information layer 32, and apush-pull method was used for the second information layer 34. Anexcellent eye pattern was observed in the reproduced signal from thefirst information layer 32. Jitters of each mark were measured, thusresulting in a standard deviation of jitters with respect to thedetection window width for the code signal of 8.1%.

[0244] EFM signals having a shortest mark length of 0.6 μm were recordedon both land portions and groove portions of the guide grooves on thesecond information layer 34. As a result of irradiation with lightmodulated between recording power of 19 mW and deletion power of 9 mW,excellent eye pattern was observed in each case. The amplitude of thelongest mark 11T was similar to that obtainable from the firstinformation layer 32. Measured jitters resulted in 8.3%. The foregoinginformation signals could be repeatedly rewritten. Note that theforegoing characteristics were equally observed from the inner portionto the outer portion of the substrate.

[0245] An experiment was performed such that the foregoing recordingmedium was allowed to stand in a hot and wet environment in which thetemperature was 80° C. and the relative humidity was 80%, for 100 houresand then the signal was similarly evaluated. As a result, change in theshape was inhibited, information could stably be reproduced, and themeasurement of jitters resulted in a satisfactory value withoutconsiderable change.

EXAMPLE 6

[0246] The specific structure of the optical information recordingmedium shown in FIG. 20 and operations for recording and reproducinginformation to and from the same will now be described.

[0247] The first substrate 41 was made of polycarbonate resin andhaving, on the surface thereof, guide grooves for tracking light beams.The thickness of the first substrate 41 was 0.58 mm and the diameter ofthe same was 120 mm. The pitch of the guide grooves formed on thesurface of the first substrate 41 was 1.48 μm, the width of the groovewas the half of the track pitch, and the depth of the groove was 50 nm.The first substrate 41 had, on the surface thereof, a ZnS-SiO₂dielectric-material layer 201 having a thickness of 110 nm, a Ge₂Sb₂Te₅recording thin film layer 202 having a thickness of 10 nm and a ZnS-SiO₂dielectric-material layer 203 having a thickness of 80 nm stackedsequentially. Thus, a reloadable first information layer 42 was formed.

[0248] The second substrate 43 was made of polycarbonate resin and had,on the surface thereof, guide grooves for tracking light beams. Thethickness of the second substrate 43 was 0.58 mm and the diameter of thesame was 120 mm. The pitch of the guide grooves formed on the surface ofthe second substrate 43 was 1.48 μm, the width of the groove was thehalf of the track pitch and the depth of the groove was 50 nm. On thesurface of the second substrate 43, there were formed a reflecting layer204 having a thickness of 100 nm and made of Al, ZnS-SiO₂dielectric-material layer 205 having a thickness of 18 nm, Ge₂ Sb₂ Te₅recording thin film layer 206 having a thickness of 25 nm and ZnS-SiO₂dielectric-material layer 207 having a thickness of 110 nm stackedsequentially. Thus, the second information layer 44 was formed.

[0249] The first substrate 41 was secured to the substrate supportsection 72 of the adhesive-bonding apparatus shown in FIG. 6. The resinapplying nozzle 64 was operated to apply the ultraviolet curing resinmaterial 80 to the upper surface of the first information layer 42. Thesecond substrate 43 was placed on the substrate support section 66 a ofthe adhesive-bonding apparatus. The elevation section 63 was operated tomove the upper-portion support section 61 downwards to be brought intocontact with the spacer 79. While applying a load to the secondsubstrate 43 from an upper position, it was irradiated with lightemitted from the light source (the ultraviolet lamp) 81. Thus, the resinmaterial 80 was hardened so that the separation layer 45 was formedbetween the first information layer 42 and the second information layer43. The mean thickness of the separation layer 45 was 40 μm with anaccuracy of ±7 μm or better at each measurement point. Note that thethickness d1 of the first substrate 41 was adapted to be 0.6 mm, whichwas the optimum base thickness of the objective lens 6 for convergingthe light beam 7. The disposition was determined such that the foregoingoptimum point was made to be the central position of the separationlayer 45. In the non-recording state (a crystal state), the reflectanceof the first information layer 42 was 19%, the transmissivity was 40%and the reflectance of the second information layer 44 was 17%.

[0250] Recording and reproducing of information to and from theforegoing recording medium were evaluated by using an optical systemcomprising a light source for emitting light having a wavelength of 680nm and an objective lens having numerical aperture (NA) of 0.6 adaptableto the optimum base thickness of 0.6 mm, the evaluation being performedat a linear velocity of 6 m/s. Note that the power of the reproducingbeam was 1 mW when the signal was evaluated. As a result, a stablefocusing operation was performed with respect to the first and secondinformation layers 42 and 44. Moreover, focus jumping betweeninformation layers was performed stably.

[0251] EFM signals having a shortest mark length of 0.6 μm were recordedon both land portions and groove portions of the guide grooves on thefirst information layer 42. As a result, an excellent eye pattern wasobserved when the recording power was 14 mW. The amplitude of thelongest mark 11T was similar to that obtainable from the firstinformation layer 42. Measured jitters resulted in excellent values suchthat it was 10.8% in the land portion and 11.3% in the groove portion.

[0252] EFM signals having a shortest mark length of 0.6 μm were recordedon both land portions and groove portions of the guide grooves of thesecond information layer 44. An excellent eye pattern was observed ateach portion when the recording power was 18 mW. Measured jittersresulted in 11.7% in the land portion and 12.1% in the groove portionwhich were lower than 13% which was one reference for reproducinginformation. Thus, it was confirmed that information couldsatisfactorily reproduced.

[0253] An experiment was performed such that the foregoing recordingmedium was allowed to stand in a hot and wet environment in which thetemperature was 80° C. and the relative humidity was 80%, for 100 houresand then the signal was similarly evaluated. As a result, change in theshape was inhibited, information could stably be reproduced, and themeasurement of jitters resulted in a satisfactory value withoutconsiderable change.

[0254] As a result, it can be said that the method according to thepresent invention is an effective method of manufacturing a recordingmedium having a plurality of information layers.

What is claimed is:
 1. An optical information recording medium havingtwo information layers and capable of reproducing information signalsfrom said information layers, or recording information signals on saidinformation layers and reproducing the recorded information signals bymeans of irradiation of light beam, said optical information recordingmedium comprising: a first substrate having, on one side thereof, atleast one selected from the group consisting of information pitscorresponding to information signals, guide grooves for tracking andsample pits; a first information layer formed on a surface of said firstsubstrate and having a predetermined transmissivity and a predeterminedreflectance with respect to said light beams; a second substrate having,on one side thereof, at least one selected from a group consisting ofinformation pits corresponding to information signals, guide grooves fortracking and sample pits; a second information layer formed on a secondsubstrate and having a predetermined reflectance; and a separation layerformed between said first information layer and said second informationlayer, said separation layer being transparent with respect to saidlight beam.
 2. The optical information recording medium according toclaim 1, wherein the thickness of said first substrate is substantiallythe same as the thickness of said second substrate.
 3. The opticalinformation recording medium according to claim 1, in which the belowmentioned formula is satisfied; R 1≈1−A 1+(2−R 2)⁻¹−{[1−A 1−(2·R2)⁻¹]²−(1−A 1)²}^(0.5) wherein R1 is a reflectance of the firstinformation layer with respect to the wavelength of the light beam toreproduce the information, A1 is an absorption ratio of the firstinformation layer, and R2 is a reflectance of the second informationlayer with respect to the wavelength of the light beam.
 4. The opticalinformation recording medium according to claim 1, wherein thereflectance of said first information layer is 25% to 40%
 5. The opticalinformation recording medium according to claim 1, wherein said secondinformation layer comprises a reflecting layer, a firstdielectric-material layer, a recording layer and a seconddielectric-material layer, which are sequentially formed on said secondsubstrate.
 6. The optical information recording medium according toclaim 1, wherein said first information layer has a absorption which issubstantially zero with respect to said light beam.
 7. The opticalinformation recording medium according to claim 1, wherein said firstinformation layer comprises at least two layers of thin films.
 8. Theoptical information recording medium according to claim 7, wherein saidfirst information layer comprises a first dielectric-material layer, arecording layer and a second dielectric-material layer, which aresequentially formed on said first substrate.
 9. The optical informationrecording medium according to claim 1, wherein both of said first andsecond substrate have information pits corresponding to informationsignals.
 10. The optical information recording medium according to claim1, wherein said first information layer is a reproduction onlyinformation layer and said second information layer is a recording andreproducing information layer.
 11. The optical information recordingmedium according to claim 1, wherein the thickness of said separationlayer is larger than the focal depth of an optical system for convergingsaid light beam and smaller than a tolerance of a base thickness allowedby said optical system.
 12. The optical information recording mediumaccording to claim 1, wherein information pits or sample pits are formedon said first and second substrates and the shapes of said pits formedon said first substrate are different from the shapes of said pitsformed on said second substrate.
 13. The optical information recordingmedium according to claim 12, wherein the width of said pit formed onsaid second substrate is larger than the width of said pit formed onsaid first substrate.
 14. The optical information recording mediumaccording to claim 1, wherein guide grooves are formed on the surfacesof said first and second substrates and the width of said guide grooveformed on said second substrate is larger than the width of said guidegroove formed on said first substrate.
 15. The optical informationrecording medium according to claim 1, wherein information pits areformed on the surfaces of said first and second substrates and thedensity of said information pits formed on said second substrate perunit area is lower than the density of said information pits formed onsaid first substrate per unit area.
 16. The optical informationrecording medium according to claim 1, wherein sample pits or guidegrooves are formed on the surfaces of said first and second substratesand the pitch of said sample pits or said guide grooves formed on saidsecond substrate is shorter than the pitch of said sample pits or saidguide grooves formed on said first substrate.
 17. The opticalinformation recording medium according to claim 1, wherein informationpits or sample pits are formed on the surfaces of the substrates and thedirections of said information pits of said first and second substratesare opposite to each other when viewed from the side from which thelight beam is made incident.
 18. The optical information recordingmedium according to claim 1, wherein said information pits, said guidegrooves or said sample pits formed on the surfaces of said first andsecond substrates are formed in a spiral shape relative to the center ofsaid first and second substrates and the shape of said spiral on saidfirst substrate and the shape of said spiral on said second substrateare the same when viewed from the side from which a light beam is madeincident.
 19. An optical information recording medium comprising: a pairof optical information recording media according to claim 1; and anadhesive-bonding layer, wherein said second substrates of said pair ofoptical information recording media are adhesive-bonded to each othervia said adhesive-bonding layer.
 20. The optical information recordingmedium according to claim 19, wherein the thicknesses of said firstsubstrates of said pair of optical information recording media aresubstantially the same and the thicknesses of said second substrates ofsaid pair of optical information recording media are substantially thesame.
 21. A method of manufacturing an optical information recordingmedium having two information layers and capable of reproducinginformation signals from said information layers or recordinginformation signals on said information layers and reproducing therecorded information signals by means of irradiating a light beam, saidmethod of manufacturing an optical information recording mediumcomprising: a first film-forming step of forming a first informationlayer having a predetermined transmissivity and a predeterminedreflectance on a substrate having, on one side thereof, at least oneselected from the group consisting of information pits corresponding toinformation signals, guide grooves for tracking and sample pits: asecond film-forming step of forming a second information layer having apredetermined reflectance on a second substrate having, on one sidethereof, at least one selected from the group consisting of informationpits corresponding to information signals, guide grooves for trackingand sample pits; a step of applying a resin layer, which is transparentwith respect to said light beams, to the upper surface of said firstinformation layer or said second information layer; an adhesive-bondingstep of locating said first information layer and said secondinformation layer to face each other and adhesive-bonding said first andsecond information layers to each other through said resin layer. 22.The method of manufacturing an optical information recording mediumaccording to claim 21, wherein the thickness of said first substrate issubstantially the same as the thickness of said second substrate. 23.The method of manufacturing an optical information recording mediumaccording to claim 21, further comprising a step of pressing said firstand second substrates from the outsides of said first and secondsubstrates.
 24. The method of manufacturing an optical informationrecording medium according to claim 21, wherein a photosetting resin isused to form said resin layer and said resin layer is irradiated withlight from a position on the outside of said first substrate so as toset said resin layer while applying pressure to the outsides of saidfirst and second substrates.
 25. The method of manufacturing an opticalinformation recording medium according to claim 21, further comprising astep of applying an adhesive-bonding layer, which is transparent withrespect to said light beam, to the upper surface of either of saidsecond substrates of a pair of optical information recording mediaobtained by said method of manufacturing an optical informationrecording medium; and an adhesive-bonding step of causing said secondsubstrates of said pair of optical information recording media to faceeach other and adhesive-bonding said second substrates to each otherthrough said resin layer.
 26. The method of manufacturing an opticalinformation recording medium according to claim 25, wherein thethicknesses of said first substrates of said pair of optical informationrecording mediums are substantially the same and the thicknesses of saidsecond substrates of said pair of optical information recording mediumsare substantially the same.
 27. An apparatus for manufacturing anoptical information recording medium comprising: a first substratesupport section for supporting a first substrate; a second substratesupport section disposed to face said first substrate support section soas to support a second substrate; an applying section for applying aresin layer to the upper surfaces of said first substrate or said secondsubstrate; a spacer disposed on a plane on which said first substratesupport section and said second substrate support section face eachother, said spacer being disposed outward from said first and secondsubstrates; and a pressing section for elevating said first substratesupport section or said second substrate support section and pressingsaid first and second substrates.
 28. The apparatus for manufacturing anoptical information recording medium according to claim 27, furthercomprising a light source disposed opposite to a surface of said firstsubstrate support section which is in contact with said first substrate,wherein said first substrate support section is made of a material whichpermits penetration of a portion of light emitted from said lightsource.
 29. The apparatus for manufacturing an optical informationrecording medium according to claim 27, wherein each of said first andsecond substrates has a central opening in the center thereof andconcentric circular or spiral information pit columns or guide grooveson either surface thereof, and a center position correction section formaking the central axes of said information pit columns or guide groovesof said first substrate and said second substrate to coincide with eachother is further provided for at least one of said first substratesupport section and said second substrate support section.
 30. Theapparatus for manufacturing an optical information recording mediumaccording to claim 27, further comprising first and second shaftsections respectively disposed on the central axes of said first andsecond substrate support sections; and first and second inner-portionguide sections each having a tapered portion having one end larger thansaid central opening of said substrate and another end smaller than saidcentral opening of said substrate, said first and second inner-portionguide sections being capable of moving along said first and second shaftsections.
 31. The apparatus for manufacturing an optical informationrecording medium according to claim 30, wherein a projecting taperedsection is formed on the leading end of one of said first shaft sectionand said second shaft section, a recessed tapered portion is formed inthe other leading end, and said projecting tapered portion and saidrecessed tapered portion determine the relative position of said firstand second substrate support sections.
 32. An apparatus for opticallyrecording and reproducing information which irradiates, with a lightbeam, an optical information recording medium having two differentinformation layers provided with at least one selected from the groupconsisting of information pits corresponding to information signals,guide grooves for tracking and sample pits so as to be capable ofreproducing information signals from said information layers, recordinginformation signals on said information layers and reproducing therecorded information signals, said apparatus for optically recording andreproducing information comprising: an objective lens to converge alight beam emitted from a light source onto said recording medium; afocus control to make the focal point of said light beam to coincidewith wither of said information layers; a tracking control forcontrolling the position of said light beam to enable said light beam tofollow said information pits, guide grooves or said sample pits; a layeridentifier for demodulating a signal for identifying said informationlayers in accordance with light reflected by or transmitted through saidinformation pits; a layer selector for selecting an information layerfrom which said information signal is reproduced or on which the s isrecorded; and a switch means for switching a method of tracking, whichis performed by said tracking control, to correspond to a result ofselection performed by said layer selector.
 33. The apparatus foroptically recording and reproducing information according to claim 32,further comprising layer comparison means for subjecting to a comparisona result of selection performed by said layer selector and a result ofidentification performed by said layer identifier, and a focus jumpingcircuit for generating a pulse voltage to move the focal point of saidlight beam between said information layers to correspond to an outputfrom said comparison means.
 34. The apparatus for optically recordingand reproducing information according to claim 33, further comprisingtracking polarity inverting means for switching the polarity of saidtracking control in synchronization with the operation of said focusjumping circuit.
 35. The apparatus for optically recording andreproducing information according to claim 32, wherein said focuscontrol comprises first focus control means for making the focal pointof said light beam to coincide with a position near said informationlayer, second focus control means permitted to be operated in a rangesmaller than that of said first focus control means, and focus switchmeans for performing switching to said second focus control means afterthe operation of said first focus control means has been completed. 36.The apparatus for optically recording and reproducing informationaccording to claim 32, wherein said focus control has at least two typesof operation conditions to be adaptable to each of said informationlayers, said focus control being arranged to select one of saidoperation conditions in accordance with a result of selection performedby said layer selector.
 37. The apparatus for optically recording andreproducing information according to claim 32, wherein said trackingcontrol has at least two types of operation conditions to be adaptableto each of said information layers, said tracking control being arrangedto select one of said operation conditions in accordance with a resultof selection performed by said layer selector.
 38. The apparatus foroptically recording and reproducing information according to claim 32,further comprising a photodetector that for performing a forcus controlcomprises a first divisional light receiving surface for receiving aportion of light reflected by said recording medium and a seconddivisional light receiving surface for receiving said reflected light onthe same plane on which said first divisional light receiving surface ispositioned, said second divisional light receiving surface receivingsaid reflected light on the outside of said first divisional lightreceiving surface.